Update build.yml

include externals in build

.github/workflows/build.yml

@@ -12,6 +12,8 @@ jobs:
     steps:
       - name: Checkout code
         uses: actions/checkout@v2
+        with:
+          submodules: recursive
       - name: Prepare ccache timestamp
         id: ccache_cache_timestamp
         shell: cmake -P {0}