github: action for git submodules, take 1

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

@@ -22,6 +22,10 @@ jobs:
     steps:
     - uses: actions/checkout@v3
 
+    - name: Update submodules
+      # update/fetch submodules (attached as top-level children of reflect/repo/)
+      run: git submodule update --init --recursive
+
     - name: Configure CMake
       # Configure CMake in a 'build' subdirectory. `CMAKE_BUILD_TYPE` is only required if you are using a single-configuration generator such as make.
       # See https://cmake.org/cmake/help/latest/variable/CMAKE_BUILD_TYPE.html?highlight=cmake_build_type
@@ -36,4 +40,3 @@ jobs:
       # Execute tests defined by the CMake configuration.
       # See https://cmake.org/cmake/help/latest/manual/ctest.1.html for more detail
       run: ctest -C ${{env.BUILD_TYPE}}
-