correctly set $this->wikiId on a running task

@drsnyder 
Looks like ```BatchRefreshLinksForTemplate``` is the only task affected by this.
Should we inject this via the parameters? It looks like it's only called [once](https://github.com/Wikia/app/blob/dev/maintenance/wikia/task_runner.php#L22) in the code base? This will eliminate the need for the global variable and make it easier to test this module.