Bug 1495395 [wpt PR 13282] - Allow all pull requests to trigger Taskcluster, a=testonly

Automatic update from web-platform-testsAllow all pull requests to trigger Taskcluster (#13282)

It was previously limited to collaborators only, i.e. members of the
web-platform-tests org.

Fixes https://github.com/web-platform-tests/wpt/issues/13280.
--

wpt-commits: 207fa91e8016fcb650e4869e0da6ef036d7671b3
wpt-pr: 13282

testing/web-platform/tests/.taskcluster.yml

@@ -1,6 +1,6 @@
 version: 1
 policy:
-  pullRequests: collaborators
+  pullRequests: public
 tasks:
   $flattenDeep:
     - $if: tasks_for == "github-push"