fix: use base repo name for project in pull requests (#954)

The rename of this repository has exposed that we rely on fork names to set `project` in PRs. This causes issues in certain circumstances, eg: if we try to fire an action in a PR where the head repository is not named the same, we end up with [the wrong treeherder routes](https://github.com/mozilla/translations/blob/f9010478a45cd40bf7ad3d0aecdd62dd281ec5d6/.taskcluster.yml#L146), which causes scope issues.

.taskcluster.yml

@@ -30,7 +30,7 @@ tasks:
           project:
               $switch:
                   'tasks_for == "github-push"': '${event.repository.name}'
-                  'tasks_for[:19] == "github-pull-request"': '${event.pull_request.head.repo.name}'
+                  'tasks_for[:19] == "github-pull-request"': '${event.pull_request.base.repo.name}'
                   'tasks_for in ["cron", "action", "pr-action"]': '${repository.project}'
           head_branch:
               $switch: