ci: Bump image
GLib 2.79.2 split out platform specific API into separate GIRs, and gjs now warns when using affected API without the new import.
However our CI image still only includes the 2.79.1 development release, so in order to allow gnome-shell to depend on a more recent glib release, bump the image to pull in a newer version.
Related: gnome-shell!3288 (merged)