Bump cache action to v4

[williamtheaker]

There don't appear to be any breaking changes between v3 and v4 as of writing.

Fixes this warning:

The following actions use a deprecated Node.js version and will be forced to run on node20: actions/cache@v3. For more info: https://github.blog/changelog/2024-03-07-github-actions-all-actions-will-run-on-node20-instead-of-node16-by-default/

https://github.com/internetarchive/bookreader/actions/runs/10930025193

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 69.24%. Comparing base (3618991) to head (8ee5c7f). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1343 +/- ## ======================================= Coverage 69.24% 69.24% ======================================= Files 59 59 Lines 5086 5086 Branches 1070 1070 ======================================= Hits 3522 3522 Misses 1537 1537 Partials 27 27 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.