Do not expose artifacts on pipeline index

Ref: #324003 (closed)

What does this MR do?

This MR does 2 things:

• Stop exposing artifacts data on the pipeline index page
• Expose has_downloadable_artifacts to determine when we need to display our download button

Why are we doing this?

Part of the ~performance improvement we are doing on the index pipeline page, we decided to load all artifacts for a given pipeline once a user click on the download button as you can see below 👇

Following up !60058 (merged) where we added a new downloadable_artifacts.json endpoint and !60058 (merged) which uses it to build up new frontend logic.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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