only run action on pull_request events if from forked repo

Author: aedwardg

.github/workflows/format-and-lint.yml

@@ -8,6 +8,7 @@ concurrency:
   cancel-in-progress: true
 jobs:
   lint:
+    if: github.event_name == 'push' || (github.event_name == 'pull_request' && github.repository != github.event.pull_request.head.repo.full_name)
     runs-on: ubuntu-latest
     strategy:
       matrix: