Add nix-flatpak flake, setup cups.

nodes/hosts/common/configuration.nix

@@ -1,14 +1,17 @@
-{ pkgs, ... }:
+{ pkgs, inputs, ... }:
 {
   imports = [
+    inputs.nix-flatpak.nixosModules.nix-flatpak
     ./programs/firefox.nix
     ./programs/general.nix
     ./programs/git.nix
     ./programs/kde.nix
     ./programs/steam.nix
     ./scripts/upgrade-diff.nix
+    ./services/cups.nix
     ./services/desktopmanager.nix
     ./services/displaymanager.nix
+    ./services/nix-flatpak.nix
     ./services/sound.nix
     ./system/bluetooth.nix
     ./system/locale.nix