ci: Use debian's gtk package instead of building our own

GTK 4.14 needs a newer glib, but we cannot build and use our own because the system glib ends up taking precedence because cargo-c doesn't set RPATHs for the plugins it builds.

The oldest GTK that supports glib 2.74 is GTK 4.10, and Debian 12 ships GTK 4.8, so let's just use the system GTK.