fix: Use correct syntax for windows

[stevehipwell]

This PR fixes the Windows support by using the correct syntax for environment variables.

[stevehipwell]

I can confirm that this is now working correctly on Windows.

~  helm plugin install https://github.com/databus23/helm-diff --verify=false
WARNING: Skipping plugin signature verification

    Directory: C:\Users\hipwells\AppData\Roaming\helm\plugins\helm-diff

Mode                 LastWriteTime         Length Name
----                 -------------         ------ ----
d----          19/11/2025    09:34                bin

Installed plugin: diff

~  helm diff --help

The Helm Diff Plugin

* Shows a diff explaining what a helm upgrade would change:
    This fetches the currently deployed version of a release
  and compares it to a local chart plus values. This can be
  used to visualize what changes a helm upgrade will perform.

* Shows a diff explaining what had changed between two revisions:
    This fetches previously deployed versions of a release
  and compares them. This can be used to visualize what changes
  were made during revision change.

* Shows a diff explaining what a helm rollback would change:
    This fetches the currently deployed version of a release
  and compares it to the previously deployed version of the release, that you
  want to rollback. This can be used to visualize what changes a
  helm rollback will perform.

Usage:
  diff [flags]
  diff [command]

Available Commands:
  completion  Generate the autocompletion script for the specified shell
  release     Shows diff between release's manifests
  revision    Shows diff between revision's manifests
  rollback    Show a diff explaining what a helm rollback could perform
  upgrade     Show a diff explaining what a helm upgrade would change.
  version     Show version of the helm diff plugin

Flags:
      --allow-unreleased                         enables diffing of releases that are not yet deployed via Helm
  -a, --api-versions stringArray                 Kubernetes api versions used for Capabilities.APIVersions
      --color                                    color output. You can control the value for this flag via HELM_DIFF_COLOR=[true|false]. If both --no-color and --color are unspecified, coloring enabled only when the stdout is a term and TERM is not "dumb"
  -C, --context int                              output NUM lines of context around changes (default -1)
      --detailed-exitcode                        return a non-zero exit code when there are changes
      --devel                                    use development versions, too. Equivalent to version '>0.0.0-0'. If --version is set, this is ignored.
      --disable-openapi-validation               disables rendered templates validation against the Kubernetes OpenAPI Schema
      --disable-validation                       disables rendered templates validation against the Kubernetes cluster you are currently pointing to. This is the same validation performed on an install
      --dry-run string[="client"]                --dry-run, --dry-run=client, or --dry-run=true disables cluster access and show diff as if it was install. Implies --install, --reset-values, and --disable-validation. --dry-run=server enables the cluster access with helm-get and the lookup template function.
      --enable-dns                               enable DNS lookups when rendering templates
  -D, --find-renames float32                     Enable rename detection if set to any value greater than 0. If specified, the value denotes the maximum fraction of changed content as lines added + removed compared to total lines in a diff for considering it a rename. Only objects of the same Kind are attempted to be matched
  -h, --help                                     help for diff
      --include-crds                             include CRDs in the diffing
      --include-tests                            enable the diffing of the helm test hooks
      --insecure-skip-tls-verify                 skip tls certificate checks for the chart download
      --install                                  enables diffing of releases that are not yet deployed via Helm (equivalent to --allow-unreleased, added to match "helm upgrade --install" command
      --kube-version string                      Kubernetes version used for Capabilities.KubeVersion
      --kubeconfig string                        This flag is ignored, to allow passing of this top level flag to helm
      --no-color                                 remove colors from the output. If both --no-color and --color are unspecified, coloring enabled only when the stdout is a term and TERM is not "dumb"
      --no-hooks                                 disable diffing of hooks
      --normalize-manifests                      normalize manifests before running diff to exclude style differences from the output
      --output string                            Possible values: diff, simple, template, dyff. When set to "template", use the env var HELM_DIFF_TPL to specify the template. (default "diff")
      --post-renderer string                     the path to an executable to be used for post rendering. If it exists in $PATH, the binary will be used, otherwise it will try to look for the executable at the given path
      --post-renderer-args stringArray           an argument to the post-renderer (can specify multiple)
      --repo string                              specify the chart repository url to locate the requested chart
      --reset-then-reuse-values                  reset the values to the ones built into the chart, apply the last release's values and merge in any new values. If '--reset-values' or '--reuse-values' is specified, this is ignored
      --reset-values                             reset the values to the ones built into the chart and merge in any new values
      --reuse-values                             reuse the last release's values and merge in any new values. If '--reset-values' is specified, this is ignored
      --set stringArray                          set values on the command line (can specify multiple or separate values with commas: key1=val1,key2=val2)
      --set-file stringArray                     set values from respective files specified via the command line (can specify multiple or separate values with commas: key1=path1,key2=path2)
      --set-json stringArray                     set JSON values on the command line (can specify multiple or separate values with commas: key1=jsonval1,key2=jsonval2)
      --set-literal stringArray                  set STRING literal values on the command line
      --set-string stringArray                   set STRING values on the command line (can specify multiple or separate values with commas: key1=val1,key2=val2)
      --show-secrets                             do not redact secret values in the output
      --show-secrets-decoded                     decode secret values in the output
      --skip-schema-validation                   skip validation of the rendered manifests against the Kubernetes OpenAPI schema
      --strip-trailing-cr                        strip trailing carriage return on input
      --suppress stringArray                     allows suppression of the kinds listed in the diff output (can specify multiple, like '--suppress Deployment --suppress Service')
      --suppress-output-line-regex stringArray   a regex to suppress diff output lines that match
  -q, --suppress-secrets                         suppress secrets in the output
      --take-ownership                           if set, upgrade will ignore the check for helm annotations and take ownership of the existing resources
      --three-way-merge                          use three-way-merge to compute patch and generate diff output
  -f, --values valueFiles                        specify values in a YAML file (can specify multiple) (default [])
      --version string                           specify the exact chart version to use. If this is not specified, the latest version is used

Additional help topics:
  diff

Use "diff [command] --help" for more information about a command.