Renintroduce pull_request in GitHub workflow with if case

The disadvantage of removing the `pull_request` as trigger event for an
GitHub workflow is that it does not trigger for pushes from a foreign
repo. To run the workflow also in this case without running the workflow
twice for PRs based on the original repo we introduce an if case that
runs the `pull_request` event only when it from a foreign repo.

.github/workflows/ci.yml

@@ -1,10 +1,12 @@
 name: ci
 
-on: [push]
+on: [push, pull_request]
 
 jobs:
 
   pre-commit:
+    # runs only on pull request when from foreign repo, prevents duplicate runs
+    if: github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name != github.event.pull_request.base.repo.full_name
 
     runs-on: ubuntu-latest
 
@@ -19,6 +21,8 @@ jobs:
     - uses: pre-commit/[email protected]
 
   tests:
+    # runs only on pull request when from foreign repo, prevents duplicate runs
+    if: github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name != github.event.pull_request.base.repo.full_name
 
     runs-on: ubuntu-latest
     timeout-minutes: 30