Avoid cross-project pipeline artifacts in gitlab-backup-cli

What does this MR do and why?

Previously gitlab-backup-cli required artifacts to be present for the db:setup pg16 job in master, but this often failed on forks because those artifacts don't exist.

We observe that the db:setup pg16 only loads the schema and does not actually insert data. We can simply download the schema directly and avoid this cross-project dependency.

References

Relates to #554732 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

See the pipeline in https://gitlab.com/gitlab-org/gitlab/-/pipelines/1922601164.