feat: Add config for generating an installation media

It can be built using the command:
nix build .#nixosConfigurations.installationMedia.config.system.build.isoImage

It contains the latest kernel ZFS supports (ZFS is included in the ISO)
and a version of Nix with flakes enabled.

config/installation_media.nix

@@ -0,0 +1,17 @@
+{ nixpkgs, ... }:
+{ config, pkgs, ... }:
+
+let
+	pkgPath = nixpkgs.outPath;
+in
+{
+	imports = [
+		"${pkgPath}/nixos/modules/installer/cd-dvd/installation-cd-graphical-gnome.nix"
+		"${pkgPath}/nixos/modules/installer/cd-dvd/channel.nix"
+	];
+
+	boot.kernelPackages = pkgs.zfs.latestCompatibleLinuxPackages;
+
+	nix.package = pkgs.nixUnstable;
+	nix.extraOptions = "experimental-features = nix-command flakes";
+}

flake.nix

@@ -16,6 +16,13 @@
 						./config/aspire_hardware.nix
 					];
 				};
+
+				installationMedia = nixpkgs.lib.nixosSystem {
+					system = "x86_64-linux";
+					modules = [
+						(import ./config/installation_media.nix inputs)
+					];
+				};
 			};
 
 			customPkgs = import ./pkg;