How to use the setoptconf.EnvironmentVariableSource function in setoptconf

def build_default_sources():
    sources = [
        build_command_line_source(),
        soc.EnvironmentVariableSource(),
        soc.ConfigFileSource((
            '.prospectorrc',
            'setup.cfg',
            'tox.ini',
        )),
        soc.ConfigFileSource((
            soc.ConfigDirectory('.prospectorrc'),
            soc.HomeDirectory('.prospectorrc'),
        ))
    ]

    return sources