authexec: lion & older only

I’m not sure this is the correct way to handle something that depends on actually running an older system. This should probably be done differently, or boneyarded entirely, or left purely in Tigerbrew, but it works even if it isn’t the nicest implementation in the world.
Ah, rats. I didn't quite know how to handle this one, so I took a wild swing at something I knew would produce a failure message. I just didn't appreciate how wide a failure message that would cause :smile_cat:.