cmd/go: ignore stderr from tool version checks

There are multiple valid reasons a tool might print to stderr.
As long as we get the expected output on stdout, that's fine.

Fixes #22588.

Change-Id: I9c5d32da08288cb26dd575530a8257cd5f375367
Reviewed-on: https://go-review.googlesource.com/76017


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>