home-manager: Extend lib instead of pkgs.lib

This will allow extending `lib` using:

```
_modules.args.lib = ...
```

Without this the extended lib is not available to home-manager modules.

modules/home-manager.nix

@@ -7,7 +7,7 @@ with lib;
 let
   cfg = config.home-manager;
 
-  extendedLib = import (home-manager-path + "/modules/lib/stdlib-extended.nix") pkgs.lib;
+  extendedLib = import (home-manager-path + "/modules/lib/stdlib-extended.nix") lib;
 
   hmModule = types.submoduleWith {
     specialArgs = { lib = extendedLib; } // cfg.extraSpecialArgs;