Bug 1614855: Run valgrind builds in AWS r=froydnj

GCE instances use Ubuntu 18.04, which is known to cause issues in
valgrind builds (Bug 1545094), while EC2 uses Ubuntu 14.04.

Move valgrind builds back to GCE to avoid perma fail of these jobs.

Differential Revision: https://phabricator.services.mozilla.com/D62552

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

taskcluster/ci/valgrind/kind.yml

@@ -28,7 +28,7 @@ jobs:
             symbol: V
             kind: build
             tier: 1
-        worker-type: b-linux
+        worker-type: b-linux-aws
         worker:
             docker-image: {in-tree: valgrind-build}
             max-run-time: 72000