nimPackages.npeg: disable threads

pkgs/development/nim-packages/npeg/default.nix

···
     rev = version;
     hash = "sha256-kN91cp50ZL4INeRWqwrRK6CAkVXUq4rN4YlcN6WL/3Y=";
   };
+  nimFlags = [ "--threads:off" ];
   meta = src.meta // {
     description = "NPeg is a pure Nim pattern matching library";
     maintainers = with lib.maintainers; [ ehmry ];