Prevent mac resource deadlock situations.

We are giving subbuilds higher priority to ensure all the tasks can
get the required resources to complete the build.

Change-Id: I17898d6706cfc2dc18a125bc35e337ddaed86bf6
Bug: https://github.com/flutter/flutter/issues/59169
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/3522
Reviewed-by: Tong Wu <wutong@google.com>
Commit-Queue: Godofredo Contreras <godofredoc@google.com>