cups: Fix scanning on hplip printers

modules/services/cups/default.nix

@@ -15,6 +15,10 @@ in {
       drivers = with pkgs; [ hplip ];
     };
 
+    services.avahi = {
+      enable = lib.mkForce true;
+      nssmdns = true;
+    };
     hardware.sane = {
       enable = true;
       extraBackends = with pkgs; [ sane-airscan hplip ];