Fix arm64 hll debsigner build context

[ibrahim halatci (ihalatci)]

Summary

Fix the arm64 debsigner Docker build context on debian-hll with one workflow-line change.

The workflow clones develop into tooling. The debsigner Dockerfile uses COPY scripts /scripts and ENTRYPOINT ["/scripts/import_and_sign"], but the old tooling-root context copies the unrelated tooling/scripts directory and omits that entrypoint. Building with tooling/dockerfiles/debsigner as the context selects the same default Dockerfile and copies the intended adjacent signing scripts.

The step remains arm64-only; amd64 behavior is unchanged. No Dockerfile, tools-version, secret, publish-gate, or platform changes are included. Bullseye repository-metadata failures and its removal are explicitly out of scope. The develop bootstrap guard from #1216 is not duplicated here.

Scope and evidence

• Exactly .github/workflows/build-package.yml, +1/-1; git diff --check passes.
• Runtime validation is limited to automatic push CI on this feature branch; no local Docker build or manual workflow dispatch/rerun.
• Existing failure evidence: https://github.com/citusdata/packaging/actions/runs/34216854535/job/102030690421
• Related: Please provide an arm64 Docker image as well citus#8612

Draft for review; this change does not alter stable publication gates or authorize overwriting already published hll versions.