rpm_builder: create the runtime packages file in $TMPDIR
    
    Instead of using workdir for the runtime packages file, use
    NamedTemporaryfile. This avoids creating a workdir for
    build-rpms/build-container where it is otherwise unneeded, and fixes
    a problem where build-container/build-container-local would error out
    because the workdir didn't exist yet.
    
    (See https://pagure.io/flatpak-module-tools/pull-request/20)