cmd/go/internal/work: add signpost to x/tools vet std test

For #79622

Change-Id: I685b8aba3e976d0ca5358d62f42320873dd1826d
Reviewed-on: https://go-review.googlesource.com/c/go/+/783380
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>