programs.zsh: set ZPLUG_HOME before loading zplug (#2987)

ZPLUG_HOME must be set before loading zplug, since otherwise it is ignored.

modules/programs/zplug.nix

@@ -45,10 +45,10 @@ in {
     home.packages = [ pkgs.zplug ];
 
     programs.zsh.initExtraBeforeCompInit = ''
-      source ${pkgs.zplug}/init.zsh
-
       export ZPLUG_HOME=${cfg.zplugHome}
 
+      source ${pkgs.zplug}/init.zsh
+
       ${optionalString (cfg.plugins != [ ]) ''
         ${concatStrings (map (plugin: ''
           zplug "${plugin.name}"${