ci: Try debian testing (trixie) for 'libzypp' backend

meson setup fails in Debian Bookworm with the following error:

Dependency libzypp found: NO found 17.25.7 but need: '>=17.31.0'

libzypp version in trixie: 17.35.9-1

tests/ci/Dockerfile-debian

@@ -1,7 +1,7 @@
-FROM debian:bookworm
+FROM debian:trixie
 
 # allow fetching source packages
-RUN echo "deb-src http://deb.debian.org/debian/ bookworm main" >> /etc/apt/sources.list
+RUN echo "deb-src http://deb.debian.org/debian/ trixie main" >> /etc/apt/sources.list
 
 # prepare
 RUN apt-get update -qq