Make rspec:coverage only download the artifacts it needs

What does this MR do?

This should make the rspec:coverage job faster.

Note that the job will now only run on gitlab-org/gitlab (no more on gitlab-org/gitlab-foss) which shouldn't be a problem since gitlab-org/gitlab is a superset of gitlab-org/gitlab-foss anyway.

Verification matrix

Current average is around 13 minutes: https://app.periscopedata.com/app/gitlab/496118/Engineering-Productivity-Sandbox?widget=7248745&udv=785399

Job	Duration
https://gitlab.com/gitlab-org/gitlab/-/jobs/476032365	5 minutes 58 seconds
https://gitlab.com/gitlab-org/gitlab/-/jobs/476245227	6 minutes 7 seconds
https://gitlab.com/gitlab-org/gitlab/-/jobs/476264605	6 minutes 40 seconds
https://gitlab.com/gitlab-org/gitlab/-/jobs/476276281	6 minutes 29 seconds

Average duration after this change should be 6 minutes 18 seconds, which is 52% faster. 🚀

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Closes #211647 (closed).