Skip runit installation if already installed

If there is an executable at the desired installation location, we can assume that runit is already installed. Unfortunately, I can't find a way to verify that the *correct version* is installed, but given that the version isn't configurable via attributes, it seems reasonable that when/if there ever is a new version of runit, this logic can be changed.

I don't know how to test this effectively, give the current minitest-based suite. If you have any thoughts I'd be happy to write some tests.

Cheers!