Wait for in-progress download by other instance with same cache

[alterNERDtive]

🗣 Description

If another instance is already downloading our file, we should wait for that download to complete instead of trying to download the same file at the same time (see #1399).

Depends on #1409 and #1410, but apparently stacked PRs are not supported across repositories yet?

Todo: This definitely needs some kind of timeout or other override condition in case a container gets stuck and the download never completes.

💭 Motivation and context

The most glaring problem should already be fixed by #1410, but it’s probably a good idea not to write to the same file multiple times anyway.

🧪 Testing

I haven’t tested this yet, just getting the drafts done before I have to head out.

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• All future TODOs are captured in issues, which are referenced
  in code comments.
• All relevant type-of-change labels have been added.
• I have read the CONTRIBUTING document.
• All relevant repo and/or project documentation has been updated
  to reflect the changes in this PR.
• Tests have been added and/or modified to cover the changes in this PR.
• All new and existing tests pass.

✅ Pre-merge checklist

• Revert dependencies to default branches.
• Finalize version.

✅ Post-merge checklist

• Add a tag or create a release.