cmd/go/internal/modload: add missing build constraint in testgo.go
That file is supposed to make unexpected dependencies on the main
module easier to diagnose in 'go test cmd/go', but I accidentally left
off the build constraint, so it was triggering outside of the test.
Updates #29097
Change-Id: I1cde3fe6c1d80add37c98a8c95ce48524ea05024
Reviewed-on: https://go-review.googlesource.com/c/153159
Run-TryBot: Bryan C. Mills <bcmills@google.com> Reviewed-by: Jay Conrod <jayconrod@google.com>