[iOS] Use Target#rake to execute Watch app launch task.

lib/motion/project/template/ios.rb

@@ -81,9 +81,7 @@ desc "Run the watch app on the sim"
 task :simulate_watch_app do
   # TODO need a way to identify a watch extension from other extensions
   extension = App.config.targets.first
-  Dir.chdir extension.path do
-    sh "env #{extension.environment_variables} rake simulator"
-  end
+  extension.rake('simulator')
 end
 
 desc "Run the simulator"