cmd/go: allow nested VCS repositories when preparing build stamp

The go command no longer reports an error when invoked in a repository
nested inside another. This check is still used by 'go get' in GOPATH
mode when locating a repository, but it's not needed when preparing
the build stamp.

Fixes #49004

Change-Id: I4ed4dcc04174d2d42da8651d47e52ab1d7d66e35
Reviewed-on: https://go-review.googlesource.com/c/go/+/356309


Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>