use github.event_name instead of github.event

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ jobs:
       - uses: actions/checkout@v2
 
       - name: create and checkout branch
-        if: github.event == 'pull_request'
+        if: github.event_name == 'pull_request'
         run: git checkout -B ${{ github.head_ref }}
 
       - name: set up JDK 1.8