[CI][VSTS] Do not run old macOS on PRs. (#10633)

fixes: https://github.com/xamarin/maccore/issues/2395

tools/devops/automation/templates/mac/stage.yml

@@ -19,8 +19,7 @@ stages:
   dependsOn:
   - build_packages
   # we need to have the pkgs built and the device sets to be ran, that is decided via the labels or type of build during the build_packages stage
-  # condition: and(succeeded(), eq (stageDependencies.build_packages.build.outputs['configuration.RunMacTests'], 'True')) 
-  condition: succeeded()
+  condition: and(succeeded(), eq(dependencies.build_packages.outputs['build.configuration.RunDeviceTests'], 'True'))
 
   jobs:
   - job: run_tests