cmd/go: skip vet when package cannot be build under "go test/vet"
If the the package cannot be built,
"go test" and "go vet" should not run the "vet" tool.
In that case only errors from the compilers will be displayed.
Fixes #26125
Change-Id: I5da6ba64bae5f44feaf5bd4e765eea85533cddd4
Reviewed-on: https://go-review.googlesource.com/123938
Run-TryBot: Yury Smolsky <yury@smolsky.by>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Bryan C. Mills <bcmills@google.com> Reviewed-by: Ian Lance Taylor <iant@golang.org>