added install xamarin step

Author: dimysa

azure-pipelines/templates/setup-dotnet.yml

@@ -15,4 +15,11 @@ steps:
   inputs:
     targetType: 'inline'
     script: |
-      dotnet workload install ios
+      dotnet workload install ios
+
+- task: Bash@3
+  displayName: Install Xamarin
+  inputs:
+    targetType: 'inline'
+    script: |
+      brew install --cask xamarin-ios