Checkout json submodule in Mac build

.github/workflows/build.yml

@@ -97,6 +97,8 @@ jobs:
     - uses: actions/checkout@v2
     - name: Checkout the Google Test submodule
       run: git submodule update --init -- lib/googletest
+    - name: Checkout the nlohmann json submodule
+      run: git submodule update --init -- lib/json
     - name: Create build directory
       run: cmake -E make_directory build
     - name: Run CMake (for Mac)