Checkout Google Test in GH workflow

I hope this works.

.github/workflows/build.yml

@@ -21,6 +21,8 @@ jobs:
       run: sudo apt-get update && sudo apt-get install -y --no-install-recommends build-essential cmake libsdl2-dev libsdl2-image-dev libsdl2-ttf-dev libsndfile1-dev libvorbis-dev libogg-dev libpng-dev libglew-dev libopenal-dev libboost-dev libboost-system-dev libboost-filesystem-dev libboost-regex-dev libphysfs-dev gettext git po4a vorbis-tools librsvg2-bin xmlstarlet
       if: matrix.container == ''
     - uses: actions/checkout@v2
+    - name: Checkout the Google Test submodule
+      run: git submodule update --init -- lib/googletest
     - name: Create build directory
       run: cmake -E make_directory build
     - name: Run CMake (for Windows using MXE)