ref test-cases fetch re-resolves from the search spec, not the committed catalog

[lewisjared]

Describe the bug

When a catalog already exists, ref test-cases fetch reruns the search rather than just fetching the datasets in the catalog.
This has implications for reproducibility.

Expected behavior

If a catalog exists everyone uses the same datasets, unless a user runs with --regen.

The fetch process should use the instance ids from the catalog instead of the general search query.