### Description of proposed changes

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### Related issue(s)

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### Testing

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- [ ] Checks pass

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### Description of proposed changes

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### Related issue(s)

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Link any related issues here. Use GitHub's special keywords if appropriate¹.
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### Testing

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- [ ] Checks pass
- [ ] (to be done by a Nextstrain team member) [Create preview PRs on downstream repositories][1].

[1]: https://github.com/nextstrain/auspice/blob/-/DEV_DOCS.md#test-on-downstream-repositories

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Add preview PRs to testing checklist
Add preview PRs to testing checklist

Since preview PRs are no longer automatically generated for each PR,
adding it to the testing checklist surfaces this feature to PR authors
who may find it useful.

Note that only Nextstrain team members with sufficient permissions can
run the workflow.
