Function run_config_init 

pub fn run_config_init(
    path: &Path,
    cp_url: String,
    ca_cert: PathBuf,
    client_cert: PathBuf,
    client_key: PathBuf,
    overwrite: bool,
) -> Result<PathBuf>

Write ~/.config/nixfleet/config.toml (or --path). Returns the absolute path so the bin can report it.