feat: use gitlab runner cache for pre-commit jobs

Closes #175 (closed)

This MR introduces caching for the pre-commit jobs. I tested this very quickly here:

https://gitlab.developers.cam.ac.uk/uis/devops/laboratory-allocator/lab-allocator/-/jobs/4228190

Unfortunately saving the cache cost us time, but overall it's quicker (and you would get the results quicker too as the extra time is now at the end instead of at the beginning).