[cbuild] Don't schedule 'google' builds for branches other than master

Change-Id: Ia4f8ff0e90872bb4a871ea60b29e793765125e01
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/179460
Commit-Queue: Karl Klose <karlklose@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>
3 files changed