Add nix-flatpak flake, setup cups.

nodes/hosts/common/services/cups.nix

@@ -0,0 +1,14 @@
+{ pkgs, ...}: {
+  # Enable CUPS to print documents.
+  services.printing = {
+    enable = true;
+    drivers = [ pkgs.cnijfilter2 ];
+  };
+
+  # Automatic discovery of printers.
+  services.avahi = {
+    enable = true;
+    nssmdns4 = true;
+    openFirewall = true;
+  };
+}