Use XDG_CONFIG_DIR instead of XDG_RUNTIME_DIR

[binarycodes]

The default user config file location is XDG_RUNTIME_DIR/ssh-keysign/config.yml
It should be XDG_CONFIG_HOMEssh-keysign/config.yml with a fallback to ~/.config/ssh-keysign/config.yml

references - https://wiki.archlinux.org/title/XDG_Base_Directory