cmd/go: make vet work with -overlay

cmd/go: make vet work with -overlay

It never worked before, an apparent oversight.
This will also make tests work, since tests run vet.

The new FIPS mode will use overlays, so this was
keeping go test from working in that mode.

Fixes #44957.

Change-Id: I3a77846b3b3f9a3a53118aaece93ee93214a36a6
Reviewed-on: https://go-review.googlesource.com/c/go/+/629199
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>