zplug: Update the path of init.zsh (#3922)

The current zplug nixpkgs puts everything under `$out/`. It pollutes the nix
profile dir.

This is a breaking change. It depends on an change of the output path in the
nixpkgs zplug package.

modules/programs/zplug.nix

@@ -47,7 +47,7 @@ in {
     programs.zsh.initExtraBeforeCompInit = ''
       export ZPLUG_HOME=${cfg.zplugHome}
 
-      source ${pkgs.zplug}/init.zsh
+      source ${pkgs.zplug}/share/zplug/init.zsh
 
       ${optionalString (cfg.plugins != [ ]) ''
         ${concatStrings (map (plugin: ''