nix: add NixOS module with usbmuxd auto-enable

[mirdukkkkk]

Builds on #658 (fixes the broken flake build). Right now there's no way to install iloader and get it actually working on NixOS in one step - a NixOS user has to know, out of band, that this app needs usbmuxd running, enable that service by hand, and separately add the package to their system. If they miss the usbmuxd step, the first symptom is just "no device found" with no obvious link back to a missing system service

This adds nixosModules.default, exposing:

programs.iloader = {
    enable = true;
    # package = <override>; # optional, defaults to this flake's own package
};

Enabling it adds the package to environment.systemPackages and sets
services.usbmuxd.enable = true, so a single option gets a user to a working
setup instead of a two-step manual dance.

Also tested in my Nix config, as in previous PR.

[mirdukkkkk]

https://github.com/NixOS/nixpkgs/blob/nixos-unstable/pkgs/by-name/il/iloader/package.nix