cmd/go: vet: revert $GOVETTOOL env var, restore -vettool flag

The environment variable is no longer necessary as we now plan to
transition to the new vet by replacing it in a single step,
and we really don't want to add more environment variables.

Fixes #28636

Change-Id: Ib85e5c0d61213b7b9f6a53d9376fec29525df971
Reviewed-on: https://go-review.googlesource.com/c/148497


Run-TryBot: Alan Donovan <adonovan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>