fix(nvim): conflict with using global packages

Now it is an error to override the pkgs if global packages are being
used.

nvim/default.nix

@@ -1,4 +1,3 @@
-{ system, unstable }:
 { lib, config, ... }:
 let
   cfg = config.jhome.nvim;
@@ -7,9 +6,8 @@ in
   imports = [ ./options.nix ];
 
   config.programs.nixvim = lib.mkMerge [
-    (import ./standalone.nix { standalone = false; })
+    (import ./standalone.nix)
     (lib.mkIf cfg.enable {
-      nixpkgs = lib.mkForce { pkgs = import unstable { inherit system; }; };
       enable = true;
       defaultEditor = lib.mkDefault true;
       jhome.nvim = cfg;