Backport v2024 <- v2025dev3: apertis-pkg-pull-updates: add mirror to snapshot.debian.org as a fallback

Sometime the main server of snapshot.debian.org is not sync which block us from downloading new packages from Debian. Thus to avoid depending on a single server add a mirror as fallback.

Signed-off-by: Dylan Aïssi dylan.aissi@collabora.com