Describe your use case and the problem you are facing

As part of some debugging I wanted to discover the location of WP-CLI's cache directory, but there appears to be no way to get this from WP-CLI itself.

The directory location is ~/.wp-cli/cache by default but can be overridden by the WP_CLI_CACHE_DIR environment variable. Ref:

Describe the solution you'd like

I'd like a command -- for example wp cli cache directory -- which exposes the location of WP-CLI's cache directory.