refactor: download sid debhelper .debs on host via pullsrc.sh instead of bundling in git

- pullsrc.sh fetches debhelper/libdebhelper-perl 14.3 (pinned DEBHELPER_SIDPKG
  in version.env, empty = auto-detect latest sid) into gitignored builddep/
- install_deps.sh installs builddep/*.deb and adds compat sed hacks for sid
  debhelper >= 13.27: rewrite Dh_Lib.pm bucket 'cp --update=none' to '-n' on
  coreutils < 9.3 (buster/bionic/bookworm), downgrade use v5.28 pragmas in
  Dh_Lib.pm/dh_assistant for perl < 5.28, handle ${tmpdir} brace form in the
  non-merged-usr hack; drop the dead sid-apt-source _DEBIAN_DEBHELPER
- Dockerfile.deps: BuildKit bind mounts instead of COPY (nothing in layers)
- CI deps-image build runs ./pullsrc.sh debhelper first to populate context

.github/workflows/build-deps-images.yml

@@ -30,6 +30,9 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v5
 
+      - name: Fetch debhelper .debs into build context
+        run: ./pullsrc.sh debhelper
+
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3