Don't run CI twice (on push AND pull request)

.github/workflows/abidiff.yaml

@@ -12,6 +12,7 @@ defaults:
 jobs:
   abi:
     runs-on: ubuntu-latest
+    if: github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name != github.event.pull_request.base.repo.full_name
     container:
       image: debian:testing