images: Use registry based caching where available
If $KOKKURI_REGISTRY is defined, add --import-cache and
--export-cache options to the buildctl command to import and export
layers, respectively.
Bug: T323149
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If $KOKKURI_REGISTRY is defined, add --import-cache and
--export-cache options to the buildctl command to import and export
layers, respectively.
Bug: T323149