ostree: Run commands in universal_newlines mode
    
    This will mean the output is returned as unicode (decoded as UTF-8).
    Thus Kobo will not have to do any decoding. This should work around
    possible errors with breaking multibyte unicode character sequences into
    different chunks.
    
    Relates: https://pagure.io/releng/failed-composes/issue/237
    Relates: https://github.com/release-engineering/kobo/issues/119
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>