AO3-6636 Move next chapter prefetch outside cache block

[domenic]

This should ensure it shows up on all chapters, including ones that have been cached recently.

This is a speculative fix and should be reviewed by someone who understands the caching mechanisms better than me, before merging. I creating this PR to make it easy to merge, in case this is the right fix.

Pull Request Checklist

• Have you read "How to write the perfect pull request"?
• Have you read the contributing guidelines?
• Have you added tests for any changed functionality? Per discussion on the ticket, no tests are needed for this feature
• Have you added the Jira issue number
  as the first thing in your pull request title (e.g. AO3-1234 Fix thing)
• Do you have fewer than 5 pull requests already open? If not, please wait
  until they are reviewed and merged before creating new pull requests.

Issue

https://otwarchive.atlassian.net/browse/AO3-6636

Purpose

Ensure speculation rules next chapter prefetching is not part of the cached content for a work, and thus shows up on all chapters.

Testing Instructions

General testing instructions for the feature are on the issue, but for this caching issue specifically, @sarken suggests testing on works which had previously been generated this change, maybe?

References

This comment https://otwarchive.atlassian.net/browse/AO3-6636?focusedCommentId=360659

Credit

Domenic Denicola, he/him

[sarken]

Closing because we went with #4670 -- thanks!