Run PR checks against feature branch head.

Author: marc0der

.github/workflows/pr.yml

@@ -6,6 +6,8 @@ jobs:
     steps:
       - name: Checkout source code
         uses: actions/checkout@v2
+        with:
+            ref: ${{ github.event.pull_request.head.sha }}
       - name: Set up JDK 11.0.11
         uses: actions/setup-java@v1
         with: