ci: Fix checkout wrong commit in auto-build (#40)

Author: Xuanwo

.github/workflows/auto-build.yml

@@ -23,6 +23,8 @@ jobs:
 
       - name: Checkout repository
         uses: actions/checkout@v2
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
 
       - name: Auto build
         run: make build