Tracking alpine-edge like debian-sid is a moving target. Usually such
rolling releases are marked as "allow_failure: true" in our CI.
However as alpine presents a musl based distro and provides useful
extra coverage lets track a release branch instead to avoid random
breakages.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20220929114231.583801-11-alex.bennee@linaro.org>