search: Do not use Cancellable.reset()

search: Do not use Cancellable.reset()

Cancellable reset API is deprecated for long time and it's quite broken.
In fact if a search provider receives a cancelled event may end up
checking if the cancellable has been cancelled, but this may not be true
anymore after a reset.

Use instead new cancellable instances so that once one is used we can
consider the previous one completed and we can forget about it.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4080>
(cherry picked from commit cead829503083658e736af5964d1d8eb3c92a6bb)