Add flag trimpath in the go build command

[pallavJha]

What this PR does / why we need it:
This PR adds the flag -trimpath in the make build and make build-cross.

make build-cross

Helm uses gox to cross compile but gox does not support the -trimpath flag. To add the flag, I've used GOFLAGS="-trimpath" env in the gox command.

make build

The flag has been directly added in the go build command.

This way the directory path, /home/circlei, is trimmed, for example,

with Helm v3.5.2

$ ./helm version
version.BuildInfo{Version:"v3.5.2", GitCommit:"167aac70832d3a384f65f9745335e9fb40169dc2", GitTreeState:"dirty", GoVersion:"go1.15.7"}
$ ./helm install abc . --dry-run --debug
install.go:173: [debug] Original chart version: ""
install.go:190: [debug] CHART PATH: /home/waterbottle/Code/linux-amd64

Error: Chart.yaml file is missing
helm.go:81: [debug] Chart.yaml file is missing
helm.sh/helm/v3/pkg/chart/loader.LoadFiles
        /home/circleci/helm.sh/helm/pkg/chart/loader/load.go:158
helm.sh/helm/v3/pkg/chart/loader.LoadDir
        /home/circleci/helm.sh/helm/pkg/chart/loader/directory.go:119
helm.sh/helm/v3/pkg/chart/loader.DirLoader.Load
        /home/circleci/helm.sh/helm/pkg/chart/loader/directory.go:41
helm.sh/helm/v3/pkg/chart/loader.Load
        /home/circleci/helm.sh/helm/pkg/chart/loader/load.go:62
main.runInstall
        /home/circleci/helm.sh/helm/cmd/helm/install.go:199
main.newInstallCmd.func2
        /home/circleci/helm.sh/helm/cmd/helm/install.go:120
github.com/spf13/cobra.(*Command).execute
        /go/pkg/mod/github.com/spf13/cobra@v1.1.1/command.go:850
github.com/spf13/cobra.(*Command).ExecuteC
        /go/pkg/mod/github.com/spf13/cobra@v1.1.1/command.go:958
github.com/spf13/cobra.(*Command).Execute
        /go/pkg/mod/github.com/spf13/cobra@v1.1.1/command.go:895
main.main
        /home/circleci/helm.sh/helm/cmd/helm/helm.go:80
runtime.main
        /usr/local/go/src/runtime/proc.go:204
runtime.goexit
        /usr/local/go/src/runtime/asm_amd64.s:1374

with this change:

$ ./helm install abc . --dry-run --debug
install.go:173: [debug] Original chart version: ""
install.go:190: [debug] CHART PATH: /home/waterbottle/Code/helm-pallav/_dist/linux-amd64

Error: Chart.yaml file is missing
helm.go:81: [debug] Chart.yaml file is missing
helm.sh/helm/v3/pkg/chart/loader.LoadFiles
        helm.sh/helm/v3/pkg/chart/loader/load.go:158
helm.sh/helm/v3/pkg/chart/loader.LoadDir
        helm.sh/helm/v3/pkg/chart/loader/directory.go:119
helm.sh/helm/v3/pkg/chart/loader.DirLoader.Load
        helm.sh/helm/v3/pkg/chart/loader/directory.go:41
helm.sh/helm/v3/pkg/chart/loader.Load
        helm.sh/helm/v3/pkg/chart/loader/load.go:62
main.runInstall
        helm.sh/helm/v3/cmd/helm/install.go:199
main.newInstallCmd.func2
        helm.sh/helm/v3/cmd/helm/install.go:120
github.com/spf13/cobra.(*Command).execute
        github.com/spf13/cobra@v1.1.1/command.go:850
github.com/spf13/cobra.(*Command).ExecuteC
        github.com/spf13/cobra@v1.1.1/command.go:958
github.com/spf13/cobra.(*Command).Execute
        github.com/spf13/cobra@v1.1.1/command.go:895
main.main
        helm.sh/helm/v3/cmd/helm/helm.go:80
runtime.main
        runtime/proc.go:204
runtime.goexit
        runtime/asm_amd64.s:1374

Special notes for your reviewer:

If applicable:

• this PR contains documentation
• this PR contains unit tests
• this PR has been tested for backwards compatibility