Consider moving keybase/kbfs_block_cache from $XDG_DATA_HOME to $XDG_CACHE_HOME

Yesterday I've uploaded few GB of files into keybase, and today I get my daily incremental backup larger by these few GB. Cache files should be stored in $XDG_CACHE_HOME, not in $XDG_DATA_HOME.

Locate where keybase/kbfs_block_cache chooses its storage directory and inspect how XDG_DATA_HOME and XDG_CACHE_HOME are handled. Confirm that cache files use XDG_CACHE_HOME rather than XDG_DATA_HOME, while the reported backup no longer includes those cache files.

Written by the indexing model from the issue text.