Per requirements.rb:

    >  XXX If the satisfy block returns a Pathname, then make sure that it
    >  remains available on the PATH. This makes requirements like
    >    satisfy { which("executable") }
    >  work, even under superenv where "executable" wouldn't normally be on the
    >  PATH.
    >  This is undocumented magic and it should be removed, but we need to add
    >  a way to declare path-based requirements that work with superenv first.

Fixes homebrew/homebrew-python#170.

Closes Homebrew/homebrew#38448.