When run on -security or -updates, act on the main distro branch

Strip off the suffix since all branches related to the upstream
Debian release need to be taken into account when pulling updates.

Without this, when the script runs on e.g. buster-security branch,
it would completely ignore the existence of any other buster branches,
leading to incorrectly applied updates.

Apertis: T7681

Signed-off-by: Andrej Shadura <andrew.shadura@collabora.co.uk>