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.

debian/salsa-ci.yml

@@ -23,6 +23,7 @@ build-debian-package:
   script:
     - 'UPSTREAM_BRANCH=$(gbp config dch.upstream-branch)'
     - git branch "${UPSTREAM_BRANCH}" "origin/${UPSTREAM_BRANCH}"
+    - git branch pristine-tar origin/pristine-tar
     - gbp dch --git-author -S -a --ignore-branch
     - mkdir -p "${LV_WORKING_DIR}/tarballs"
     - gbp export-orig --tarball-dir="${LV_WORKING_DIR}/tarballs"