Version command doesn't show client version

[jcogilvie]

What happened?

The crossplane-diff version command was not reporting the correct version for the crossplane-diff binary.
Instead, it was attempting to report the version from the upstream github.com/crossplane/crossplane
dependency, which resulted in either an empty version string or the wrong version being displayed.

Root cause:

• crossplane-diff was importing the version command directly from
  github.com/crossplane/crossplane/v2/cmd/crank/version
• The Earthfile was attempting to inject version information into
  github.com/crossplane/crossplane/internal/version.version using ldflags
• This approach doesn't work because you cannot inject ldflags into vendored/downloaded dependencies
• The variable name CROSSPLANE_VERSION was overloaded - used both for the upstream Crossplane image tag (e.g.,
  release-1.20) and the crossplane-diff binary version, causing confusion

Expected behavior:
When running crossplane-diff version --client, it should display the actual version of the crossplane-diff
binary (e.g., v0.4.0 or v0.0.0-{timestamp}-{git-hash} for dev builds).

Actual behavior:
The version command displayed an empty string or incorrect version information.

How can we reproduce it?

Before the fix:

1. Clone the crossplane-diff repository at a commit before the fix
2. Build the binary: go build -o crossplane-diff ./cmd/diff
3. Run: ./crossplane-diff version --client
4. Observe that the version is empty or incorrect

Or using Earthly:

1. Build with Earthly: earthly +build
2. Run: ./_output/bin/darwin_arm64/crossplane-diff version --client (adjust path for your platform)
3. Observe the version output is empty or incorrect

What environment did it happen in?

Crossplane version: n/a