Bug 1651542 - Force font cache generation for ubuntu1804-test docker image. r=jmaher

Fixing the font cache issue the 3rd time now by just running "fc-cache" as the worker user should hopefully fix it for real, and no longer be regressed by some docker image changes.

The Puppeteer jobs will keep the 15s startup timeout for Firefox, so that we could detect another fallout again.

Differential Revision: https://phabricator.services.mozilla.com/D87745

taskcluster/docker/recipes/ubuntu1804-test-system-setup.sh

@@ -183,3 +183,6 @@ apt-get -y autoremove
 apt-get clean
 apt-get autoclean
 rm -f "$0"
+
+# Pre-create caches for user "worker"
+su --command "fc-cache -vf" worker