lua51Packages.nfd: fix module not loading

As this (issue)[https://github.com/NixOS/nixpkgs/issues/295022] complains, it wasn't
finding symbols properly.

I traced it to the (migration)[https://github.com/NixOS/nixpkgs/pull/288669] from buildLuarocksPackage.extraVariables to
buildLuarocksPackage.luarocksConfig.variables where someone forgot to
add .variables.

It now launches the dialogue properly.

pkgs/development/lua-modules/nfd/default.nix

···
   ];
   knownRockspec = "lua/nfd-scm-1.rockspec";
 
-  luarocksConfig.LUA_LIBDIR = "${lua}/lib";
+  luarocksConfig.variables.LUA_LIBDIR = "${lua}/lib";
   nativeBuildInputs = [ pkg-config ];
 
   buildInputs = lib.optionals stdenv.isDarwin [ AppKit ];