Fix plugin update crashing after the rows are refreshed

Plugin::refreshRows() drops the Sushi connection, and only a newly
constructed model boots it again. updatePlugin() called refresh() on the
instance it was given, which queries through the dropped connection and
fails with "Call to a member function query() on null". Load the plugin
again with findOrFail(), like the plugin jobs already do.

app/Services/Helpers/PluginService.php

@@ -321,7 +321,7 @@ class PluginService
         $this->downloadPluginFromUrl($downloadUrl, $plugin->id);
 
         Plugin::refreshRows();
-        $plugin = $plugin->refresh();
+        $plugin = Plugin::findOrFail($plugin->id);
 
         $this->installPlugin($plugin, false);