package-source-builder: ci-buildpackage: Never create orig tarballs

If for any reason `git-buildpackage` fails to detect the orig tarball
extracted from the `pristine-lfs` branch it tries to generate it using
`pristine-tar` or `git archive`.

This is not desirable, we definitely prefer erroring out in case
something goes wrong as we always want to rely on `pristine-lfs` to
ensure that the orig tarballs are bit-identical to the upstream ones.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>