cmd/coordinator: use gitmirror on Kubernetes instead of local Docker

Updates golang/go#18817

Change-Id: I1142f079f3748fefcf9fd1d5749e58db86afbda6
Reviewed-on: https://go-review.googlesource.com/36807
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
4 files changed