gs-appstream: Check the cancellable in gs_appstream_do_search()

The search can be cancelled while the user types the search term,
but the gs_appstream_do_search() doesn't check the cancellable
at all, which means it can do the search, which will be discarded
anyway. Early return avoids unneeded work.

Closes https://gitlab.gnome.org/GNOME/gnome-software/-/issues/1989