Subflakes should provide their wares as outputs, so wire up the pkgs
flake to reflect that.
Due to the unstable nature of flakes, updating the root flake doesn't
currently update the subflake lock file. Therefore, add additional
logic to flk update script in order to do this behind the scenes.
Nix is now pulled in from the "nix" registry flake in order for users
to take advantage of improvements to the UI since its last update in
nixpkgs.
* Update the readme to pull from cachix first, so users don't have to
build nix.
* Pull `nixFlakes` from override for slightly newer ui, and consistency
for users who change the nixos flake ref to something newer.
