Meta: nix flake does not work on macOS

[ADKaster]

The nix build instructions and nix files have never been suitable for a macOS installation of nixpkgs.

We should update the nix files and documentation to allow nixpkgs users on macOS to build Ladybird and SerenityOS in a nix shell or nix env.

Assuming that any such macOS contributors exist, that is :)