Break up the build into two stages, performing basic tests with poky and
isar on the two kas container types before uploading them. The tests
consist of building zlib-native with poky and cowsay for x86 bullseye
with Isar.
Note that the ordering "build kas", "test kas", "push kas",
"build kas-isar" is important because "build kas-isar" will not use the
locally built and imported kas:next image and rather pull the previous
one from the registry.
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
