Redundant mathlib caching

[austinletson]

One area for improvement is that currently the action caches the mathlib cache generated by lake exe cache get which seems redundant and takes up significant space in the GitHub cache.

One potential fix for this would be to exclude certain subfolders of .lake from the cache. This behavior of the cache action may be relevant.

[oliver-butterley]

Did you figure out what is happening with the github cache?

In use on my projects I don't find that it increases the speed of successive runs, I think github cache is never actually used. As yet I didn't investigate what was happening since possibly someone else already knows the solution.

[austinletson]

I haven't investigated this any further since creating the initial issue.