ci: revert running on the source repo of the PR

The permissions require a tweak to the default settings of the token, so
we need to run it under our own repo.

.github/workflows/label-pr-size.yml

@@ -1,6 +1,6 @@
 name: Label PRs based on size
 
-on: [pull_request]
+on: [pull_request_target]
 
 jobs:
   add_pr_size_label: