bug: ghostty package should be nullable for unsupported platforms

[mhanberg]

Are you following the right branch?

• My Nixpkgs and Home Manager versions are in sync

Is there an existing issue for this?

• I have searched the existing issues

Issue description

I believe that the package option for the ghostty module should be "nullable", as ghostty is not supported on macos through nixpkgs, but you can still use the hm module manage your config while having it installed via another method.

Currently, you get an error about macos being broken, as it tries to install it.

Maintainer CC

@HeitorAugustoLN

System information

- system: `"aarch64-darwin"`
 - host os: `Darwin 24.1.0, macOS 15.1.1`
 - multi-user?: `yes`
 - sandbox: `no`
 - version: `nix-env (Nix) 2.24.10`
 - nixpkgs: `/Users/mitchell/.nix-defexpr/channels/nixpkgs`