Init submodules in CI build

Author: GDYendell

.github/workflows/cpp.yml

@@ -10,6 +10,9 @@ jobs:
         uses: actions/checkout@v6
         with:
           fetch-depth: 0
+      - name: Init submodules
+        run: |
+          git submodule update --init --recursive
       - name: Build eiger-detector
         run: |
           git config --global --add safe.directory `pwd` && \