sky_tools artifact cache should match directory structure of build dirs

[abarth]

From https://github.com/flutter/tools/issues/43:

Cached binary artifacts should be laid out in a structure that matches the build structure to make it easy to swap cached artifacts from the cloud with a developer's local build. The current cache layout doesn't do this.

[abarth]

I think this is a non-goal.

[github-actions]

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