Checkout submodules. [ci skip]

.github/workflows/ci-main.yml

@@ -17,6 +17,8 @@ jobs:
     steps:
       # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
       - uses: actions/checkout@v2
+        with:
+          submodules: "true"
 
       # Runs a set of commands using the runners shell
       - name: Run a multi-line script