Support both Taskcluster deployments

Part of https://bugzilla.mozilla.org/show_bug.cgi?id=1574648

etc/taskcluster/mock.py

@@ -44,6 +44,7 @@ sys.modules["taskcluster"] = sys.modules[__name__]
 sys.dont_write_bytecode = True
 os.environ.update(**{k: k for k in "TASK_ID TASK_OWNER TASK_SOURCE GIT_URL GIT_SHA".split()})
 os.environ["GIT_REF"] = "refs/heads/auto"
+os.environ["TASKCLUSTER_ROOT_URL"] = "https://taskcluster.net"
 import decision_task
 
 print("\n# Push:")