cmd/updatestd: print stderr in runOut when command fails
cmd/updatestd: print stderr in runOut when command fails

Stderr is much more likely to contain useful information than
stdout in the unexpected case of the command failing. Show it.

For golang/go#55067.

Change-Id: Ice85e957bb7d9104da475d7616519bbfbae105e3
Reviewed-on: https://go-review.googlesource.com/c/build/+/431357
Auto-Submit: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@google.com>

golang/build,golang/build,golang/build,golang/build