feat!(pm): move printing out to redhead

In our ongoing quest to move away from configuration.nix, this commit
moves printing over to redhead (which to-my-knowledge is the only
machine we want to print on). To store printer configurations (and be
useful in the first place) redhead needs the CUPS directory persisted,
so let's do that...

packetmix/systems/personal/configuration.nix

···
   ...
 }:
 {
-  # Enable CUPS to print documents.
-  services.printing.enable = true;
-
   # Enable touchpad support (enabled default in most desktopManager).
   # services.xserver.libinput.enable = true;
 

packetmix/systems/redhead/printing.nix

···
+# SPDX-FileCopyrightText: 2025 Collabora Productivity Limited
+#
+# SPDX-License-Identifier: MIT
+
+{ pkgs, ... }:
+{
+  services.printing.enable = true;
+  clicks.storage.impermanence.persist.directories = [ "/var/lib/cups" ];
+}