ci: bump DEBIAN_TAG

The previous commit requires wayland-protocols 1.18. Bump DEBIAN_TAG to
re-generate the Debian image and get the wayland-protocols update.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
3 jobs for xdg-output-done in 7 minutes and 48 seconds (queued for 6 seconds)
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