fix CI (hopefully)

.github/workflows/validate_operations.yml

@@ -30,10 +30,10 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
-        with:
-          submodules: recursive
-          github-server-url: https://chef.heaplab.deib.polimi.it/git
+        run: |
+          git clone --depth 1 --recurse-submodules --branch ${GITHUB_REF_NAME} \
+            https://chef.heaplab.deib.polimi.it/git/${GITHUB_REPOSITORY}.git \
+            ${GITHUB_WORKSPACE}
 
       - name: Install system dependencies
         run: |