CI: Check out submodules before build

.github/workflows/cmake-multi-platform.yml

@@ -46,6 +46,8 @@ jobs:
 
     steps:
     - uses: actions/checkout@v3
+      with:
+        submodules: true
 
     - name: Set reusable strings
       # Turn repeated input strings (such as the build output directory) into step outputs. These step outputs can be used throughout the workflow file.