https://nixos.org/manual/nix/unstable/command-ref/new-cli/nix3-registry.html

https://github.com/NixOS/flake-registry
https://raw.githubusercontent.com/NixOS/flake-registry/master/flake-registry.json

equivalents:
    $ nix shell nixpkgs#cowsay --command cowsay Hi!
    $ nix shell github:NixOS/nixpkgs#cowsay --command cowsay Hi!

nix
    registry
        list
            system flake:comma path:/nix/store/...
            system flake:cowsay path:/nix/store/...
            ...
            system flake:flake path:/nix/store/...
            ...
            global flake:flake-parts github:herculesci/flake-parts
            global flake:flake-utils github:numtide/flake-utils
            ...
            global flake:nix github:NixOS/nix
            global flake:nixos-hardware github:NixOS/nixos-hardware
            global flake:nixos-homepate github:NixOS/nixos-homepage