use $XDG_CONFIG_HOME for default config file path

It would be great it rustscan could use XDG_CONFIG_HOME on Linux to respect the XDG Base Directory spec and avoid cluttering the user's home directory. I believe it would make sense to check for an existing file here, then fallback to ~/.rustscan.toml if it does not exist. It may also make sense to support this on MacOS if the environment variable is set, as that seems to be a commonly requested feature in other software. I'm not sure of an equivalent on Windows.