Fix arm64 Citus release debsigner build context

[ibrahim halatci (ihalatci)]

Summary

Correct the stable Citus arm64 debsigner build context on all-citus with exactly one changed workflow line:

-        run: docker build -t citusdata/packaging:debsigner -f tooling/dockerfiles/debsigner/Dockerfile tooling
+        run: docker build -t citusdata/packaging:debsigner tooling/dockerfiles/debsigner

The workflow clones develop into tooling. The debsigner Dockerfile uses COPY scripts /scripts and ENTRYPOINT /scripts/import_and_sign. The old tooling-root context copies tooling/scripts, which does not contain that entrypoint. Using tooling/dockerfiles/debsigner selects the same default Dockerfile and copies the correct adjacent scripts.

The identical wrong context caused /scripts/import_and_sign not found in nightly CI. Refs citusdata/citus#8612.

Scope

Only .github/workflows/build-package.yml changes (+1/-1). The amd64 path, Dockerfiles, tools pin, secrets, platform matrix, and publication gates are unchanged. The develop bootstrap guard from #1216 is already merged and requires no port to this branch.

Bullseye's repository-metadata failure is explicitly out of scope. No release publication is dispatched, and no existing PackageCloud packages are overwritten or deleted. This feature branch remains publish-gated.

Validation

• git diff --check passed.
• Confirmed the file matches the original with only the approved replacement, including preserved indentation and line endings.
• Used only automatically triggered push CI for head 555499ac62ebdb2dda84899de32da1aed2d8f8bf; no local Docker build, manual dispatch, or rerun.

Automatic head run	Final outcome
Build Package 34240822123	Failure: 11 package jobs succeeded, 3 failed; downstream package-test trigger skipped.
Nightly 34240822451	Success: all 6 amd64 jobs succeeded.

All four non-Bullseye arm64 builder and debsigner-image steps succeeded. Jammy arm64 and Bookworm arm64 completed successfully: both logs show DEB signing finished successfully. and Package publishing skipped since current branch is not equal to all-citus.

Trixie arm64 and Noble arm64 failed before signing because existing build-output validation rejects dpkg-shlibdeps: warning: diversions involved - output may be incorrect. These are separate package-build warnings, not the missing signer entrypoint. Bullseye arm64 failed at the excluded builder-image stage. All 9 stable amd64 jobs succeeded. No changes were added for these out-of-scope failures.