Split cki_pipeline.yml into sensible chunks

AC:

• use local includes and !reference to divide the shell code into sensible chunks (https://docs.gitlab.com/ee/ci/yaml/#reference-tags)
• add support to the shell-in-yaml-linter for that (https://gitlab.com/cki-project/cki-lib/-/blob/main/cki_lib/gitlab_yaml_shellcheck.py)