Update dotnet.yml

.github/workflows/dotnet.yml

@@ -18,6 +18,9 @@ jobs:
     steps:
       # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
       - uses: actions/checkout@v2
+        with:
+          submodules: recursive
+    
       - name: setup-msbuild
         uses: microsoft/setup-msbuild@v1