here, to speed up subsequent retraces.

        If permanent_rootdir is True, then the sandbox created from the
        downloaded packages will be reused, to speed up subsequent retraces.

        If architecture is given, the sandbox will be created with packages of
        the given architecture (as specified in a report's "Architecture"
        field). If not given it defaults to the host system's architecture.

        If origins is given, the sandbox will be created with apt data sources
        for foreign origins.

        If install_deps is True, then the dependencies of packages will also
        be installed.

        Return a string with outdated packages, or None if all packages were
        installed.

        If something is wrong with the environment (invalid configuration,
        package servers down, etc.), this should raise a SystemError with a
        meaningful error message.
        r