Add action option to skip cache prune

[polothy]

Hello, wondering if it was possible to have the option to skip pruning the cache prior to saving it.

The prune completely removes all downloads and I saw in here that this is normally wanted behavior because re-downloading is often faster than downloading the cache. But in my particular use case, I'm using a private registry with large packages and we pay egress fees to download. So, GitHub cache is actually cheaper.

If this isn't a feature you want to have in this action, that is totally okay, I'll just continue to use actions/cache@v4.

Cheers and thanks!