Bug 1340564: use an attribute to identify signed tasks; r=Callek

This is a more robust approach than using substring matching on task labels.

As an optimization, this simply avoids creating balrog tasks for unsigned beets
using only-for-attributes, rather than omitting them in a transform.

MozReview-Commit-ID: 8MNOxu0WgXo

taskcluster/taskgraph/transforms/beetmover.py

@@ -222,6 +222,7 @@ def make_task_description(config, jobs):
 
         attributes = {
             'nightly': dep_job.attributes.get('nightly', False),
+            'signed': dep_job.attributes.get('signed', False),
             'build_platform': dep_job.attributes.get('build_platform'),
             'build_type': dep_job.attributes.get('build_type'),
         }