cmd/go: add hint to GOVCS error messages

[jayconrod]

If GOVCS prevents the go command from downloading a repository, the user is shown a message like:

go get rsc.io/nonexist.git/hello: GOVCS disallows using git for public rsc.io/nonexist.git

This message should recommend reading go help vcs or the module reference documentation, which should explain the GOVCS environment variable.

[gopherbot]

Change https://golang.org/cl/282713 mentions this issue: cmd/go: add hint to read 'go help vcs' to GOVCS errors