Vet help prints only to stdout.

Fixes #31885

Change-Id: If6089a371fa8e21828eba2e23cddd2d19fb69e8a
Reviewed-on: https://go-review.googlesource.com/c/go/+/176617Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>