Reintroduce pull_request in GitHub workflow with if case (#84)

GH workflows will now also be run for PRs from forks.

.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