It seems XDG_CONFIG_HOME is hard-coded on macOS, if I set an env variable $XDG_CONFIG_HOME = ~/.config eget still doesn't use ~/.config/.eget.toml, whereas ~/.eget.toml works fine...
I don't want to use $HOME/Library/Application Support/eget/.eget.toml and the XDG spec itself does not specify what macOS should use: https://specifications.freedesktop.org/basedir-spec/latest/