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.

CCBUG: 423818