make CI build against branch head instead of merge (#831)

.github/workflows/ci-reusable.yml

@@ -32,6 +32,7 @@ jobs:
         uses: actions/checkout@v4
         with:
           submodules: recursive
+          ref: ${{ github.event.pull_request.head.sha }}
 
       - name: Setup Nimbus Build System
         uses: ./.github/actions/nimbus-build-system

.github/workflows/ci.yml

@@ -48,6 +48,7 @@ jobs:
         uses: actions/checkout@v4
         with:
           submodules: recursive
+          ref: ${{ github.event.pull_request.head.sha }}
 
       - name: Setup Nimbus Build System
         uses: ./.github/actions/nimbus-build-system