Taskcluster: run tasks for PRs from everyone

These tasks should be untrusted, since they run before review.
The `repo:github.com/servo/servo:pull-request` role determines what scopes
(permissions) those tasks have.

https://tools.taskcluster.net/auth/roles/repo%3Agithub.com%2Fservo%2Fservo%3Apull-request

.taskcluster.yml

@@ -1,7 +1,7 @@
 version: 1
 policy:
   # https://docs.taskcluster.net/docs/reference/integrations/taskcluster-github/docs/taskcluster-yml-v1#pull-requests
-  pullRequests: collaborators
+  pullRequests: public
 tasks:
   - $if: 'tasks_for == "github-push"'
     then: