ci: cache frontend build across jobs
ci: ensure caches are saved with zstd
work around https://github.com/actions/cache/issues/1169
ci: require unit tests for remote cacher
- prevents unnecessary runs in case the unit tests already fail
- starts the integration tests about 2 minutes earlier
- should give some overall speedup to the CI run, because the long integration tests are run and finish earlier, and the cacher tests should still usually finish in time
- does not save any computing resources, just provides quicker results when runners are not under high load
