What does the "failed to compute cache key" error mean on an Earthly build running on a persistent CI node, specifically due to the inability to obtain the index's state?

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The "failed to compute cache key" error message means that the Earthly build system was unable to compute the cache key for a particular step. This error can occur if the index's state cannot be obtained, which may happen if there is an issue with the persistent CI node's connection to the Earthly server, or if there is a problem with the index itself.

If the index's state cannot be obtained, the cache key cannot be calculated accurately, and the build will fail with this error message. To resolve the issue, you may need to investigate the connection between the persistent CI node and the Earthly server, or contact Earthly support if there is an issue with the index.