ci: init opendbc submodule after sparse checkout to resolve car.capnp symlink

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/cereal_validation.yaml

@@ -32,6 +32,9 @@ jobs:
         with:
           sparse-checkout: cereal
 
+      - name: Init sunnypilot opendbc submodule
+        run: git submodule update --init --depth 1 opendbc_repo
+
       - name: Checkout upstream openpilot cereal
         uses: actions/checkout@v6
         with:
@@ -40,6 +43,10 @@ jobs:
           sparse-checkout: cereal
           ref: "refs/heads/master"
 
+      - name: Init upstream opendbc submodule
+        working-directory: upstream_openpilot
+        run: git submodule update --init --depth 1 opendbc_repo
+
       - name: Install uv
         run: pip install uv