fix escaping of powershell variable

As this is nested in a powershell variable $command, it must be escaped
otherwise it is evaluated when the variable is created, giving an error that
"The term 'True' is not recognized as the name of a cmdlet, function,
script". This prevented using a puppet.working_directory on Windows.

plugins/provisioners/puppet/provisioner/puppet.rb

@@ -149,7 +149,7 @@ module VagrantPlugins
           command = "#{facter}puppet apply #{options}"
           if config.working_directory
             if windows?
-              command = "cd #{config.working_directory}; if ($?) \{ #{command} \}"
+              command = "cd #{config.working_directory}; if (`$?) \{ #{command} \}"
             else
               command = "cd #{config.working_directory} && #{command}"
             end