Expose installer as a module

[n8henrie]

It would be nice to expose the iso-configuration to flake outputs, which will make it significantly easier for end users to create their own customized installer images.

For example, in my case I have been greatly enjoying Asahi NixOS on ZFS root, and so having ZFS support in the installer makes things much easier!
With this PR, users like myself can easily create their own installer image with ZFS support (and git, for flakes and rsync), like so:

packages.x86_64-linux.zfs-installer =
    (nixpkgs.lib.nixosSystem {
      inherit system;
      pkgs = import nixpkgs {
        crossSystem.system = "aarch64-linux";
        localSystem.system = system;
        overlays = [ nixos-silicon.overlays.default ];
      };
      modules = [
        nixos-silicon.outputs.nixosModules.apple-silicon-installer
        { hardware.asahi.pkgsSystem = system; }
        (
          { pkgs, ... }:
          {
            boot.supportedFilesystems.zfs = true;
            networking.hostId = "AAAAAAAA";
            environment.systemPackages = with pkgs; [
              git
              rsync
            ];
          }
        )
      ];
    }).config.system.build.isoImage;

There may be a way to use some overrides with the existing installer-bootstrap output, but if so, I haven't yet figured it out: https://discourse.nixos.org/t/custom-asahi-installer

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/custom-asahi-installer/69034/6