[feat] flake: prepare for 24.05

We add an `unstable` overlay (available through `pkgs.unstable`) and pin
nixpkgs to the `nixos-24.05` branch. Neovim is still kept in unstable (I
want to live in the bleeding edge c:).

system/default.nix

@@ -22,7 +22,6 @@ in {
     ./options.nix
     ./gui
     stylix.nixosModules.stylix
-    # FIXME(https://github.com/danth/stylix/issues/216): Must configure stylix
     {stylix = import ./stylix-config.nix {inherit config pkgs;};}
   ];