build: Disable GTK+ demo app by default

Given that it's not usable yet.
2 jobs from wip/hadess/gtk-demo-crash in 5 minutes and 30 seconds (queued for 8 minutes and 39 seconds)
Status Job ID Name Coverage
  Build
passed #86139
build

00:04:08

 
  Test
passed #86140
flatpak master

00:01:21