Update to actions/cache v4 (GitHub actions: transition from Node 16 to Node 20)

[valassi]

I saw a warning in one of the CIs, eg https://github.com/madgraph5/madgraph4gpu/actions/runs/9124168744:

This links to https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/

I imagine that I need to use cache v4 (and that I no longer need to gh install cache manually)
https://github.com/actions/cache

PS: the blog entry also suggests we may need to update self-hosted github runners at some point...

[valassi]

This is now done in PR #798, I updated to cache v4 (which I assume will fix this)