In TFS 2012 you'll need to edit the build workflow to do this. You need to add an InvokeProcess activity at some point after the workflow runs your tests. See the ALM Ranger Build Guidance for more info on how to customize build workflows: http://vsarbuildguide.codeplex.com/
If you were using TFS 2013 the new TfvcTemplate.12.xaml includes the ability to specify a powershell file to run right from the build definition (no workflow customization required)