dig: tiny code cleanup

https://github.com/NixOS/nixpkgs/pull/252097#issuecomment-1697244637

pkgs/top-level/all-packages.nix

···
 
   bind = callPackage ../servers/dns/bind { };
   dnsutils = bind.dnsutils;
-  dig = bind.dnsutils // {
-    meta = bind.dnsutils.meta // {
-      mainProgram = "dig";
-    };
-  };
+  dig = lib.addMetaAttrs { mainProgram = "dig"; } bind.dnsutils;
 
   bird = callPackage ../servers/bird { };