Test caching can cause an incorrect test failure when the vet step result
is reused, leading to not printing a vet command line at all.
Avoid that with -a (we are also using -n so no real work is done).
Fixes one failing case in 'go test cmd/go' on my Mac.
Change-Id: I028284436b1ecc77145c886db902845b524f4b97
Reviewed-on: https://go-review.googlesource.com/c/go/+/718181
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Matloob <matloob@golang.org> Reviewed-by: Michael Matloob <matloob@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>