ci: Update to Debian 13 (trixie)

Switch containers from Debian 12 (bookworm) to Debian 13 (trixie).

Trixie ships LLVM 19 by default, so we no longer need to add LLVM repos
to install llvm-19.

Notably, trixie also uses Python 3.13.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/6994
Signed-off-by: Valentine Burley <valentine.burley@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35853>

.gitlab-ci/common/init-stage1.sh

@@ -29,7 +29,7 @@ fi
 
 # Set the time so we can validate certificates before we fetch anything;
 # however as not all DUTs have network, make this non-fatal.
-for _ in 1 2 3; do sntp -sS pool.ntp.org && break || sleep 2; done || true
+for _ in 1 2 3; do ntpdig -sS pool.ntp.org && break || sleep 2; done || true
 
 # Create a symlink from /dev/fd to /proc/self/fd if /dev/fd is missing.
 if [ ! -e /dev/fd ]; then