cupsd: Allow major upgrades of gutenprint with cups-genppdupdate. (#129337)

nixos/modules/services/printing/cupsd.nix

···
 
             ${optionalString (containsGutenprint cfg.drivers) ''
               if [ -d /var/lib/cups/ppd ]; then
-                ${getGutenprint cfg.drivers}/bin/cups-genppdupdate -p /var/lib/cups/ppd
+                ${getGutenprint cfg.drivers}/bin/cups-genppdupdate -x -p /var/lib/cups/ppd
               fi
             ''}
           '';