cmd/gitmirror: push to prod namespace
cmd/gitmirror: push to prod namespace

Overlooked this -- the set-image command doesn't use a YAML file so
needs to specify the namespace explicitly.

Change-Id: I3602dbb31afc27df6087207b57b081975024021d
Reviewed-on: https://go-review.googlesource.com/c/build/+/347411
Trust: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Run-TryBot: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

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