nixos/zsh: remove `lib.lib`

This fails my NixOS configuration:

```
error: attribute 'lib' missing

at /nix/store/ninrqc3pblnmqgh489cbr9rq5pijcpd6-nixpkgs-src/nixos/modules/programs/zsh/zsh-syntax-highlighting.nix:90:7:

89| programs.zsh.interactiveShellInit =
90| lib.lib.mkAfter (lib.concatStringsSep "\n" ([
| ^
91| "source ${pkgs.zsh-syntax-highlighting}/share/zsh-syntax-highlighting/zsh-syntax-highlighting.zsh"
```

Changed files
+1 -1
nixos
modules
+1 -1
nixos/modules/programs/zsh/zsh-syntax-highlighting.nix
···
];
programs.zsh.interactiveShellInit =
-
lib.lib.mkAfter (lib.concatStringsSep "\n" ([
+
lib.mkAfter (lib.concatStringsSep "\n" ([
"source ${pkgs.zsh-syntax-highlighting}/share/zsh-syntax-highlighting/zsh-syntax-highlighting.zsh"
] ++ lib.optional (builtins.length(cfg.highlighters) > 0)
"ZSH_HIGHLIGHT_HIGHLIGHTERS=(${builtins.concatStringsSep " " cfg.highlighters})"