fixes https://github.com/TryGhost/DevOps/issues/120
- we've seen errors in CI where the unit tests for the apps folder are
being run concurrently, but the build assets are not present so
multiple builds are happening for the same folder
- this leads to errors because Vite tries to cleanup or build folders
at the same time
- this should fix that by eagerly building the apps and caching the
output so unit tests run quicker
617d3a6ffc