How to pin nix registry nixpkgs to release channel

[wmertens]

@bew

To configure the user registry, it is stored in ~/.config/nix/registry.json and I configure it with a home manager module of my own:
https://github.com/bew/dotfiles/blob/99d174e92d2f6d0d50f326e875198d37b84fff28/nix-home/modules/nix-registry.nix

I think home-manager now has this as part of the nix module, see home-manager/modules/misc/nix.nix at 0232fe1b75e6d7864fd82b5c72f6646f87838fc3 · nix-community/home-manager · GitHub