[cron] Add a new recipe to trigger cron jobs with an input commit

The luci-scheduler doesn't provide an input commit to builds triggered
by a cron job. This recipe is intended to be used by a cron builder that
triggers a set of builders with the current Dart SDK tip of tree as
input commit. Setting the input commit ensures that the recipes can
handle cron triggered builds like regular builds and that luci-milo
displays builds correctly on consoles.

Change-Id: Ic650e30b6edcff7c483d2c15119fe0ede9e4f10e
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/146600
Reviewed-by: William Hesse <whesse@google.com>
4 files changed