ci: run only once

.github/workflows/test.yml

@@ -6,6 +6,9 @@ jobs:
   test:
     runs-on: ubuntu-latest
 
+    # We want to run on external PRs, but not on our own internal PRs as they'll be run by the push to the branch.
+    if: github.event_name == 'push' || github.event.pull_request.head.repo.full_name != github.repository
+
     steps:
       - uses: actions/checkout@v2