Added singularity to main pipeline

updated install.py script consistent with new repo structure.

Also rolled back ACTS/Gaudi/python to their v2.3.x versions as this is the last version that works with our downstream software. We can re-upgrade after we set this stable baseline with the new CI implementation.

I think the old api was being used and was removed when the gitlab
server was updated.

Note gitlab is moving to graphql api in the next version
https://docs.gitlab.com/ee/api/graphql/

- don't install a root wrapper script (sometimes breaks root when working in the
container)
- gaudi tag  stuck on gcc 9