fix(neovim): guard NixOS module to Linux for nix-darwin compatibility 🔧

- wrap Neovim module in `lib.mkIf pkgs.stdenv.isLinux` (no-op on macOS)
- add `lib` to module args
- fix Lua message var: use `config_file` instead of undefined `file`
- keep existing plugins and customRC on Linux unchanged

nixos/hosts/macos.nix

@@ -62,6 +62,7 @@
     fzf
     cowsay
     lolcat
+    neovim
   ];
 
   # Keep for darwin as well (tracks defaults across upgrades)