Please stop barebacking `$HOME` on Linux

[Ramblurr]

On Linux, for over 20 years, it has been bad manners for tools to drop their config/state/cache dirs into $HOME

Flyctl does not follow the XDG spec, but at least provides a FLY_CONFIG_DIR env var superfly/flyctl#2785

Please, follow the Freedesktop XDG base directory standard: https://specifications.freedesktop.org/basedir/latest/

The simplest fix is simply use $XDG_CONFIG_HOME/sprites as the config dir. If $XDG_CONFIG_HOME doesn't exist then and the platform is linux, then default to $HOME/config.

Congrats on the launch btw, sprites is really cool.