nixpkgs entry should default to nixpkgs-unstable ref

[lilyball]

The nixpkgs entry right now is github:NixOS/nixpkgs. This means that a reference to the nixpkgs flake will default to the master branch, meaning anyone who uses this may get packages that haven't yet been built by hydra. This is surprising and something of a footgun.

The registry should instead set it to github:NixOS/nixpkgs/nixpkgs-unstable. This mirrors the default <nixpkgs> channel set up by the Nix installer. Anyone who wants the master branch can reference it as nixpkgs/master. And similarly, other branches like nixpkgs/nixos-21.11 will still work as they do today.

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/future-of-channels-and-channels-nixos-org-in-a-flakes-world/11563/10