zsh: avoid IFD while sourcing prezto

modules/programs/zsh.nix

@@ -815,10 +815,7 @@ in
             ))
 
             (mkOrder 900 (
-              (optionalString cfg.prezto.enable (
-                builtins.readFile "${cfg.prezto.package}/share/zsh-prezto/runcoms/zshrc"
-              ))
-              + (lib.concatStrings (
+              (lib.concatStrings (
                 map (plugin: ''
                   if [[ -f "$HOME/${pluginsDir}/${plugin.name}/${plugin.file}" ]]; then
                     source "$HOME/${pluginsDir}/${plugin.name}/${plugin.file}"

modules/programs/zsh/prezto.nix

@@ -586,6 +586,11 @@ in
           ''}
           ${cfg.extraConfig}
         '';
+
+        programs.zsh.initContent = lib.mkOrder 850 ''
+          # Load prezto
+          source ${cfg.package}/share/zsh-prezto/runcoms/zshrc
+        '';
       }
     ]
   );