ci: create a 'complete-pr' task to block PRs from landing

Author: ahal

taskcluster/config.yml

@@ -38,3 +38,8 @@ workers:
             implementation: docker-worker
             os: linux
             worker-type: linux-gcp
+        succeed:
+            provisioner: built-in
+            implementation: succeed
+            os: none
+            worker-type: succeed

taskcluster/kinds/complete/kind.yml

@@ -0,0 +1,15 @@
+transforms:
+  - taskgraph.transforms.from_deps
+
+kind-dependencies:
+  - check
+  - test
+
+tasks:
+  pr:
+    description: Ensures required PR tasks have completed
+    run-on-tasks-for: ["github-pull-request"]
+    worker-type: succeed
+    from-deps:
+      group-by: all
+      set-name: false