Add a test for csharp, run in CI

.github/workflows/build.yaml

@@ -111,6 +111,11 @@ jobs:
         path: gadgets/macos/download
       name: Cache gadgets
 
+    - name: 'Install .NET Core SDK 3.1'
+      uses: actions/setup-dotnet@v1.7.2
+      with:
+        dotnet-version: 3.1
+
     - run: vim --version
       name: 'Print vim version information'