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.
Fixeshomebrew/homebrew-python#170.
ClosesHomebrew/homebrew#38448.
