Caching uses too much memory

[patrickersing]

In TrixiShallowWater.jl I noticed error messages from julia-actions/cache@v1 that old caches could not be deleted. This seems to be solved by setting some permissions explicitly as suggested in https://github.com/julia-actions/cache.

I was wondering if this should be also done in Trixi.jl or why this is not frequently encountered in Trixi.jl. Looking at the CI runs there I saw that this happens rarely as here, but most of the times it says "No existing caches found".

The reason for this is probably that our caches get deleted by github because we use too much memory. Looking at the current cache status, a single test job saves a cache of around 1GB. If I am correct it means that even a single CI run creates around 20GB of caches, so twice our cache limit.

Github will then start to delete old caches until it reaches the 10GB limit. Besides that, from what I have seen all CI jobs can still load a cache (probably also because the limit is not enforced so strictly), so it doesn't seem to cause problems right now.

I still think it would be good to keep an eye on this and find some strategies to reduce our generated cache data.