Allow turning off go's buildvcs

[marckhouzam]

What this PR does / why we need it

There are situations where people want to build the CLI without having git present. This requires turning off buildvcs in the go build command.

This PR allows to turn off buildvcs by setting the BUILDVCS=false environment variable.

Which issue(s) this PR fixes

Fixes N/A

Describe testing done for PR

# By default -buildvcs=true
$ make prepare-builder
cd cmd/plugin/builder && go build -buildvcs=true -o /Users/kmarc/git/tanzu-cli/bin/builder .

# can override -buildvcs=false
$ BUILDVCS=false make prepare-builder
cd cmd/plugin/builder && go build -buildvcs=false -o /Users/kmarc/git/tanzu-cli/bin/builder .

Release note

Allow turning off go's `buildvcs` when building the CLI.

Additional information

Special notes for your reviewer

[vuil]

lgtm, thanks!