CI configuration: Move cache to build stage where it belongs.

Since ad228bda, caching has been applied to the "pages" stage instead of the "build" stage. I think this was a mistake since the purpose of the cache was to avoid re-downloading APKs that haven't changed and that work is now done as part of the "build" stage. For evidence, see this pipeline run before ad228bda, which got a "304 Not Modified" on kiwix and didn't download it. In contrast, these two recent pipeline runs (1, 2) both downloaded kiwix even though it was at version 3.4.3 both times, so presumably unchanged.

I'm not familiar with the CI configuration syntax, but I think this change has a good enough chance of being correct that I decided to go ahead and submit a MR instead of an issue.

Note to anyone reviewing this: To run this repository's pipeline on the MR, it may be enough to copy my branch to this repository and run the pipeline from this page. As long as the pipeline is run on the same commit, I hope the success will show up on this MR and unblock it. Of course, if you prefer to file a separate MR (like !35 (merged)), that's fine.