Nix 2.7 Flake schema change

[c0c0n3]

Nix 2.7 made some changes to the Flake schema:

• https://discourse.nixos.org/t/nix-2-7-0-released/18072

Switch now to the new schema to avoid pain down the line. It's not a train smash as far as I can tell, but Nixie libs, pkgs and docs (especially the wiki pages) will need updating.