[COOK-3330] Only adds the helper on windows (fixes crashes on *nix)

This fixes a critical bug (anything that includes powershell (even if it's not added as a recipe) won't run at all on *nix, including berkshelf.)

Should fix this ticket: https://tickets.opscode.com/browse/COOK-3330 

Caveats: I don't have a windows system to test on - can't guarantee functionality stays as desired on Win, but it should.