salsa-ci: Create local pristine-tar branch

Now that we enforce the presence of the upstream signature, not
having a local pristine-tar branch will result in

  $ gbp export-orig --tarball-dir="${LV_WORKING_DIR}/tarballs" --pristine-tar
  gbp:info: All Orig tarballs 'libvirt_6.0.0.orig.tar.xz' found at '/builds/libvirt-team/libvirt/debian/output/tarballs'
  gbp:warning: Pristine-tar branch "pristine-tar" not found
  gbp:info: Creating /builds/libvirt-team/libvirt/debian/output/tarballs/libvirt_6.0.0.orig.tar.xz
  gbp:error: Can not find requested upstream signature for archive 'libvirt_6.0.0.orig.tar.xz' in pristine tar commit.

Even before, gbp reported the same warning about the pristine-tar
branch being missing, although it still managed to produce an
upstream tarball somehow.