Fix schedulling of tasks.

We were giving higher priority to drone tasks leading to a problem where the
main tasks were never completing.

Change-Id: Ib6ac617a13da3ca5d6c30067019bf948c310db7b
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/5540
Reviewed-by: Keyong Han <keyonghan@google.com>
Commit-Queue: Godofredo Contreras <godofredoc@google.com>