CI: use buildbot container for building

CI: use buildbot container for building

Instead of using a fresh Linux installation which is setup every time
use the Buildbot container which is used for our own Buildbot
infrastructure, too.
While at it also tidy up the workflow to make it more consistent with
other workflow.

Signed-off-by: Paul Spooren <mail@aparcar.org>
Co-Developed-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
(cherry picked from commit 3b23227d43ec720f810e6e261945530f7bc549f0)