Schedule async weekend build of index for job traces for SaaS only

What does this MR do and why?

This MR schedules a temporary index to be built asynchronously over the weekend. We want to build an index for the job trace records in ci_job_artifacts that need to have their expire_at timestamps removed.

Doing this could potentially take a long time and block deployments on gitlab.com, so we should do it over the weekend.

This MR is being split of from database maintainer review in an existing MR: !79547 (comment 850220280)

This is a second version of !81332 (closed) set up to run only for GitLab SaaS

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