nixd: add meta.mainProgram

Signed-off-by: Paul Meyer <[e-mail]>

pkgs/development/tools/language-servers/nixd/default.nix

···
     changelog = "https://github.com/nix-community/nixd/releases/tag/${version}";
     license = lib.licenses.lgpl3Plus;
     maintainers = with lib.maintainers; [ inclyc Ruixi-rebirth marsam ];
+    mainProgram = "nixd";
     platforms = lib.platforms.unix;
   };
 }