Investigate return code of job artifacts API

Failing build during CE-to-EE merge: https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/32221973

We changed the spec to expect 404, but @grzesiek needs to investigate if this is indeed the now-correct status code.

On a related note, if we extracted the API implementation to job_artifacts.rb, the spec probably should have moved with it.