PluginLoader should update lookup_path_list if data_dir is not ".". (#7447)

Should update lookup_path_list instead of extra_paths

It looks like a problem introduced by dee72a431c (diff-b3b0e0e90ea7b43a9d97a1c7572acef3f1dc7d1d2f75fe634345a1b841c624fa).

frontend/pluginloader.lua

@@ -66,7 +66,9 @@ function PluginLoader:loadPlugins()
     else
         local data_dir = require("datastorage"):getDataDir()
         if data_dir ~= "." then
-            G_reader_settings:saveSetting("extra_plugin_paths", { data_dir .. "/plugins/" })
+            local extra_path = data_dir .. "/plugins/"
+            G_reader_settings:saveSetting("extra_plugin_paths", { extra_path })
+            table.insert(lookup_path_list, extra_path)
         end
     end