Bug 1917014 - Ensure python-test tasks use the public pypi server, r=taskgraph-reviewers,jmaher

Differential Revision: https://phabricator.services.mozilla.com/D221246

taskcluster/kinds/source-test/python.yml

@@ -22,6 +22,8 @@ task-defaults:
                 max-run-time: 3600
                 env:
                     PATH: "/usr/local/bin:/bin:/sbin:/usr/bin:/usr/sbin"
+                    # ensure we don't use internal pypi for these tasks
+                    PIP_INDEX_URL: https://pypi.python.org/simple/
             default:
                 max-run-time: 3600
     use-python: default