action/compile: Update checkout to use PR head

Signed-off-by: Audun Korneliussen <audun.korneliussen@nordicsemi.no>

Author: nordic-auko

.github/workflows/compile-samples.yml

@@ -58,6 +58,8 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v4
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
 
       - name: Setup environment
         # The toolchain should be installed already, but it might be under a sha instead of tag.