nixos/flannel: Refactor to use config.networking.fqdnOrHostName

nixos/modules/services/networking/flannel.nix

···
         Needed when running with Kubernetes as backend as this cannot be auto-detected";
       '';
       type = types.nullOr types.str;
-      default = with config.networking; (hostName + optionalString (domain != null) ".${domain}");
-      defaultText = literalExpression ''
-        with config.networking; (hostName + optionalString (domain != null) ".''${domain}")
-      '';
+      default = config.networking.fqdnOrHostName;
+      defaultText = literalExpression "config.networking.fqdnOrHostName";
       example = "node1.example.com";
     };