Add local cache folder to avoid repeated downloads of the same entries from the remote cache

[vlsi]

This would improve build performance, reduce S3 costs, and it would reduce network bandwidth usage at the developer's machine.

Relevant Gradle issue: gradle/gradle#13928

[jGleitz]

As discussed in the linked issue, this is not an issue as long as the local build cache is enabled. Let’s re-purpose this issue to remove the recommendation in the README.md to disable the local build cache. This recommendation is against what Gradle recommends, and I also don’t find the reasoning convincing:

Local build cache is dangerous as it might produce inconsistent results
in case developer modifies files while the build is running

To trigger this problem, a developer would need to do the modification between when gradle calculates its hashes and when the task actually runs. That is a pretty short window. Also: the local build cache is enabled by default now and it’s not known to cause widespread issues.

Hence, I propose to remove the recommendation from the README.md. We will then get the behaviour @vlsi wants: the local build cache will be populated and preferred over the remote cache.