ci: Ensure forever callbacks do not leak

The current implementation of forever callbacks leaks memory, GTK merged the first usage of forever callbacks after merging GJS support so this is now causing a failure in our CI leak sanitizer.

!647 (merged) added initial support for notified callbacks which did not expose a destory callback, gtk!3796 (merged) merged the first usage of this support. The current implementation leaks callbacks which breaks CI with newer versions of GTK.

Example broken pipeline: https://gitlab.gnome.org/GNOME/gjs/-/jobs/1699216