base_ref bustage: extend the [11:] hack to action/cron

(cherry picked from commit 6bd8e99)

.taskcluster.yml

@@ -82,7 +82,10 @@ tasks:
                           else: ${event.ref}
                       else:
                           $if: 'tasks_for in ["cron", "action"]'
-                          then: '${push.branch}'
+                          then:
+                            $if: '${push.branch}[:11] == "refs/heads/"'
+                            then: {$eval: 'push.branch[11:]'}
+                            else: '${push.branch}'
           base_sha:
               $if: 'tasks_for == "github-push"'
               then: '${event.before}'