On a user level there are no expected changes, other than being able
to update submodules and libraries from a main repository at a detached
HEAD situation (which did not work before).
On the infrastructure level of things this moves us closer to ability
to use the main make_update.py for the buildbot update-code stage, and
to remove the update-code section from the pipeline_config.yaml.
The initial idea of switching make_update to the pipeline config did
not really work, causing duplicated work done on blender side and the
buildbot side. Additionally, it is not easy to switch make_update.py
to use pipeline_config.yaml because the YAML parser is not included
into default package of Python.
There will be few more steps of updates to this script before we can
actually clean-up the pipeline_config: the changes needs to be also
applied on the buildbot side to switch it to the actual make_update.
Switching buildbot to the official make_update.py allows to much more
easily apply the submodules change as per #104573.
