Thanks @tomprince!

The nixpkgs fork already contains all of the changes our overlays were
responsible for - but as direct nixpkgs changes instead.

Rely on upstream package to supply typing and challenge-bypass-ristretto as
necessary.  Simplify our expression (approximately pointfree style) so we
don't have to know about any other arguments that might be passed or accepted.

At least, I suppose it was incorrect.  It won't build anywhere it has not
already built.  I don't know how it built where it already built.