debuginfod-client: default to XDG cache. PR25502: debuginfod client should default to XDG cache Location of client cache now defaults to $XDG_CACHE_HOME/debuginfod_client. If XDG_CACHE_HOME is not set then fallback to $HOME/.cache/debuginfod_client. Also maintain backwards compatibility with the previous default path- if $HOME/.debuginfod_client_cache exists, use that instead of the new defaults. Signed-off-by: Aaron Merey <[email protected]>