Add option to read environment from cache but not set new cache

[jonashaag]

Use case: In PRs, we might want to not create any new cache entries so as to not burn through the cache limits so quickly.

Similar to https://github.com/hendrikmuhs/ccache-action/blob/572582a14cd1c7a54f51762befec013fff6ba6e5/action.yml#L20-L22

[rhshadrach]

This would be very helpful for the pandas CI, especially given that GitHub is now enforcing cache limits more accurately. If we were able to only create cache entries from the main branch, it could be shared among the dozens of PRs that get updated daily. By creating caches in feature branches, evictions happen much more frequenlty.