Consider new sandbox configuration in cache keys.

This adds SandboxConfig.get_unique_key() to delegate
the cache key contribution to the SandboxConfig.

Further, this moves over the OS and Machine Architecture
parts of cache key calculation into SandboxConfig.get_unique_key(),
removing the comment which speaks of later delegating this
part of cache key calculation to sandboxes.

Cache key calculation algorithm is backward compatible,
and so the artifact version remains unchanged.