[fenix] Try to fix build bustage from https://github.com/mozilla-mobile/fenix/issues/1321. (https://github.com/mozilla-mobile/fenix/pull/1538)

This just looks wrong.  I'm concerned that this was green as a PR,
because it suggests that we were signing with an incorrect
`worker_type` there; but here we are -- let's see if it addresses the
issue.  If not, we'll back the whole of https://github.com/mozilla-mobile/fenix/issues/1321 out.

automation/taskcluster/lib/tasks.py

@@ -271,7 +271,7 @@ class TaskBuilder(object):
         }
 
         return self._craft_default_task_definition(
-            worker_type='mobile-signing-dep-v1' if signing_format == 'dep' else 'mobile-signing-v1',
+            worker_type='mobile-signing-dep-v1' if signing_type == 'dep-signing' else 'mobile-signing-v1',
             provisioner_id='scriptworker-prov-v1',
             dependencies=[assemble_task_id],
             routes=routes,