Bug 1589706: [firefox-ci] Use AWS Provider-based decision worker; r=Callek

Differential Revision: https://phabricator.services.mozilla.com/D50357

--HG--
extra : moz-landing-system : lando

.taskcluster.yml

@@ -96,8 +96,8 @@ tasks:
                   name: "Decision Task for cron job ${cron.job_name}"
                   description: 'Created by a [cron task](https://tools.taskcluster.net/tasks/${cron.task_id})'
 
-        provisionerId: "aws-provisioner-v1"
-        workerType: "gecko-${repository.level}-decision"
+        provisionerId: "gecko-${repository.level}"
+        workerType: "decision"
 
         tags:
           $if: 'tasks_for == "hg-push"'