appearance-install-theme: Update GTK icon cache after installing

Otherwise Appearance dialog shows warning for the icon themes.
2 jobs for !53 with install-theme in 2 minutes and 58 seconds (queued for 3 seconds)
latest detached
Status Job ID Name Coverage
  Build
passed #14244
build

00:01:12

 
  Distcheck
passed #14245
distcheck

00:01:46