nixos/plasma6: install discover if flatpak is enabled (#342852)

nixos/modules/services/desktop-managers/plasma6.nix

···
         spectacle
         ffmpegthumbs
         krdp
+      ] ++ lib.optionals config.services.flatpak.enable [
+        # Since PackageKit Nix support is not there yet,
+        # only install discover if flatpak is enabled.
+        discover
       ];
     in
       requiredPackages