disable ksyscoca rebuilding in appstream runner
same as 5a648c56 for the service runner.
Kservice internally will hold a KDirWatch instance through KSycoca and that instance has an inotify instance. Since inotify instances have a per-user limit we mustn't be wasteful with them. Since our runner threads do not actually need automatic ksyscoca reloading (reloading on-demand as we use ksyscoca is perfectly reasonable) we need to explicitly disable this feature so the internal inotify instance can be freed.