flatpak-indexer: fixes for flatpak-indexer-build
    
    * Output the build to the correct imagestream - it was outputing
      to the flatpak-indexer-tardiff stream from a cut-and-paste mistake
    * Add a ImageChange trigger for the from: image - we want to trigger
      a rebuild if the ubi8/python-38 image changes by itself, even if
      the flatpak-indexer-tardiff image where we get the tardiff binary from
      isn't rebuilt.
    
    Signed-off-by: Owen W. Taylor <otaylor@fishsoup.net>