Bug 1304484 - Add back gecko vcs environment variables for desktop-test tasks, r=gps

This was regressed by bug 1296397 which stopped setting the GECKO_HEAD_REPOSITORY and
the like. It wasn't caught because the task doesn't actually depend on those environment
variables except when using the interactive loaner.

This patch supports vcs checkout regardless of whether the task runs from source or not.

MozReview-Commit-ID: CDxdG8G6JDd

--HG--
extra : rebase_source : 8a3d380c86bbfabe6d76854fd8af8e794e75970e

taskcluster/taskgraph/transforms/tests/make_task_description.py

@@ -198,10 +198,13 @@ def docker_worker_setup(config, test, taskdesc):
         '--chown', '/home/worker/workspace',
     ]
 
+    # Support vcs checkouts regardless of whether the task runs from
+    # source or not in case it is needed on an interactive loaner.
+    docker_worker_support_vcs_checkout(config, test, taskdesc)
+
     # If we have a source checkout, run mozharness from it instead of
     # downloading a zip file with the same content.
     if test['checkout']:
-        docker_worker_support_vcs_checkout(config, test, taskdesc)
         command.extend(['--vcs-checkout', '/home/worker/checkouts/gecko'])
         env['MOZHARNESS_PATH'] = '/home/worker/checkouts/gecko/testing/mozharness'
     else: