modules/performance: don't combine filesPlugin into plugin pack

It's expected that user may want to override some runtime files in its
own config directory. Do not combine it into plugin pack to avoid
collisions.

modules/top-level/files/default.nix

@@ -93,5 +93,8 @@ in
           ]
         ) extraFiles}
       '';
+
+      # Never combine user files with the rest of the plugins
+      performance.combinePlugins.standalonePlugins = [ config.filesPlugin ];
     };
 }