Update README nix shell URLs for gitlab host switch

Because ghc.nix is now meant to live here, I presume these URLs should be used. It took me some time to figure out the right incantations, so I'm making this PR in case it saves others time. I tried the new URLs myself, and they seem to work, with the caveat that I am not a direnv user, so I did not try that one.

The flake spec is here, if it's helpful.

Thanks!