fix: build PR head, not some random merge commit

Author: smlx

.github/workflows/build.yaml

@@ -6,6 +6,8 @@ jobs:
     steps:
     - name: Checkout
       uses: actions/checkout@v3
+      with:
+        ref: ${{ github.event.pull_request.head.sha }}
     - name: Install Go
       uses: actions/setup-go@v3
       with: