nixpkgs: Add aarch64-darwin as buildable target

[shazow]

Couple of people verified that wifitui works with aarch64-darwin so we can open it up in nixpkgs.

https://github.com/NixOS/nixpkgs/blob/master/pkgs/by-name/wi/wifitui/package.nix#L35

- platforms = lib.platforms.linux;
+ platforms = lib.platforms.linux ++ lib.platforms.darwin;