Don't leave dangling pointers in the GcalEvent cache

The cache has an unowned pointer to the GcalEvent's UID. We need to
update the cache with the GcalEvent's new UID to avoid use-after-free.

Fixes #434
3 jobs for mcatanzaro/#434-gnome-3-32 in 5 minutes and 6 seconds (queued for 1 second)
Status Job ID Name Coverage
  Flatpak
passed #416872
flatpak

00:04:40

 
  Review
passed #416873
review

00:00:25

manual #416874
allowed to fail manual
stop_review