[cron] Avoid scheduling cron builds for already built revisions

Change-Id: Ie490041db5fa0493203cfd72dc1e6e35e64e991c
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/162880
Commit-Queue: Karl Klose <karlklose@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>
4 files changed