CI: Also clone submodule

Author: felsenhower

.github/workflows/soundness_check.yaml

@@ -11,6 +11,8 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v4
+        with:
+          submodules: 'true'
 
       - name: Install Valgrind
         run: sudo apt-get update && sudo apt-get install -y valgrind