Allow nix-flake-dispatch to work with Lix

[johnhamelink]

This change to the nix-is-24 predicate allows nix.el to parse nix --version when the underlying nix executable is actually lix.

[ed9w2in6]

I tried and your fix works but I don't think Lix's version has the same meaning as Nix's version.
Correct me if I am wrong on this.

If I am correct on this then I think the right way to do this should be setting a "compatible" version for
nix-version. In my config I did this:

(setq nix-version "nix (Nix) 2.4")

Which I believe is a more sane way to handle it, as it supports any other forks (determinate, tvix, lix, etc.)

In that case work need to be done on add more documentation and hints to guide users to make such a config.

[johnhamelink]

@ed9w2in6 thanks for this - I haven't been using lix for a few months as I had a blocking issue I didn't have time to get to the bottom of, but I will try out your suggestion in the coming days as I switch back.