ci: prevent running jobs twice for push and pull_request events

.github/workflows/main.yml

@@ -12,8 +12,43 @@ concurrency:
   cancel-in-progress: true
 
 jobs:
+  should_run:
+    name: Should run
+    runs-on: ubuntu-latest
+    outputs:
+      should_run: ${{ steps.check.outputs.should_run }}
+    steps:
+    - name: Check whether workflow should run
+      env:
+        GH_TOKEN: ${{ github.token }}
+      id: check
+      run: |
+        if [ "${GITHUB_EVENT_NAME}" = "pull_request" ]; then
+          echo "This run was triggered by a pull request event"
+          OUTPUT="should_run=true"
+          echo "Setting '$OUTPUT'"
+          echo "$OUTPUT" >> $GITHUB_OUTPUT
+          exit
+        fi
+        BRANCH_NAME="${GITHUB_REF#refs/heads/}"
+        PR_COUNT=$(gh api "repos/${GITHUB_REPOSITORY}/pulls?state=open" \
+                     | jq --arg branch "$BRANCH_NAME" '.[] | select(.head.ref == $branch) | .number' \
+                     | wc -l)
+        if [ $PR_COUNT -gt 0 ]; then
+          echo "There is an associated pull request for branch $BRANCH_NAME"
+          OUTPUT="should_run=false"
+          echo "Setting '$OUTPUT'"
+          echo "$OUTPUT" >> $GITHUB_OUTPUT
+        else
+          echo "There is no associated pull request for branch $BRANCH_NAME"
+          OUTPUT="should_run=true"
+          echo "Setting '$OUTPUT'"
+          echo "$OUTPUT" >> $GITHUB_OUTPUT
+        fi
   pre-commit:
     name: Run `pre-commit`
+    needs: should_run
+    if: fromJSON(needs.should_run.outputs.should_run)
     runs-on: ubuntu-latest
     timeout-minutes: 5
     steps: