use dotnet build

Author: Charles Roddie

.github/workflows/Test.yml

@@ -87,7 +87,7 @@ jobs:
       with:
         submodules: 'recursive'
     - run: dotnet restore CSharpMath.Ios.slnf
-    - run: msbuild CSharpMath.Ios.Tests
+    - run: dotnet build CSharpMath.Ios.Tests
     - name: Run tests
       env:
         IOS_SIM_NAME: iPhone 11 # https://github.com/actions/virtual-environments/blob/master/images/macos/macos-10.15-Readme.md#installed-simulators