Jenkinsfile: build LXC container only once for architecture

Do not need to build OSTree and ostree-based rootfs for LXC container
for every board. Build it only once for architecture instead.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>
Reviewed-by: Emanuele Aina <emanuele.aina@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D7866