Bug 1546858 - Force the wpt manifest job to rebuild the manifest, r=Aryx

If we use the downloaded manifest then any bug that leads to an error in the manifest
may be propogated forward. Instead force the manifest to be built from scratch in CI.

Differential Revision: https://phabricator.services.mozilla.com/D28809

taskcluster/ci/source-test/wpt-manifest.yml

@@ -20,7 +20,7 @@ upload:
         using: run-task
         command: >
             cd /builds/worker/checkouts/gecko
-            && ./mach wpt-manifest-update --config testing/web-platform/wptrunner.ini
+            && ./mach wpt-manifest-update --config testing/web-platform/wptrunner.ini --no-download
             && tar -cvzf manifests.tar.gz -C testing/web-platform/ meta/MANIFEST.json mozilla/meta/MANIFEST.json
     worker:
         artifacts: