Use uv cache in CI

[ValerianRey]

It seems that highspy doesn't have wheels for python 3.14 and it takes quite a lot of time to build (about 2 minutes on my computer). So our CI is much slower for python 3.14 because it needs all this extra time to build the same package every time. The advantage of having using the uv cache is probably much greater now, because of that. I think this could reduce CI duration back to ~1 minute.

[ValerianRey]

Actually, uv caching already works most of the time. It doesn't cache everything (they deliberately choose to save by default only a pruned cache, because it's empirically faster to re-download most packages). However, it does cache libraries that have to be built after download, such as highspy.

The problem is that github caches are per-branch. So for the first time we run the checks on a PR, no caches will exist for this branch and it each runner will have to build every package that doesn't come with wheels (like highspy). There is an exception to this: actions from a branch can also read from the cache of the main branch. We save to this cache for the default install (python 3.14 ubuntu with full deps) after each push to main through the build & deploy docs action. We also save to this global cache for every configuration (all python versions and os) daily, during the daily tests run. So, we may have up to 24 hours of slowdown on new PRs whenever dependencies change, until the next daily tests run that will save to the global cache. If this is a problem, we can manually run the tests on main branch using a workflow dispatch.