Bug 1580622: Build geckodriver on push; r=nalexander

Differential Revision: https://phabricator.services.mozilla.com/D46229

--HG--
extra : moz-landing-system : lando

taskcluster/ci/toolchain/geckodriver.yml

@@ -7,6 +7,7 @@ job-defaults:
     worker-type: b-linux
     worker:
         max-run-time: 1800
+    run-on-projects: ['trunk']
     run:
         script: build-geckodriver.sh
         toolchain-artifact: public/build/geckodriver.tar.xz