Add convenience package build workflow for cloudberry-backup

[tuhaihe]

Introduce a manually-triggered GitHub Actions workflow that builds portable tar.gz packages for Apache cloudberry-backup from its official source release tarball, and tests them against Apache Cloudberry built from its own official source release.

Key design decisions:

• cloudberry-backup binaries are built on Rocky 8 (glibc 2.28) to maximise compatibility across Linux distributions.
• Cloudberry is built per-platform from its source release tarball (~8 min), avoiding dependence on pre-built Docker images or external DEB/RPM repositories.
• Build-and-test are fused into one job per platform: after Cloudberry compiles, gpdemo spins up directly in the same container, eliminating artifact packaging, transfer, and a separate test job.
• Only .sha512 checksums are generated; .asc signatures remain a release manager local step.

closes: #ISSUE_Number

---

Change logs

Describe your change clearly, including what problem is being solved or what document is being added or updated.

Contributor's checklist

Here are some reminders before you submit your pull request:

• Make sure that your Pull Request has a clear title and commit message. You can take the Git commit template as a reference.
• Learn the code contribution and doc contribution guides for better collaboration.
• Make sure that CICD workflow is successful.
• List your communications in the GitHub Issues or Discussions (if has or needed).
• Feel free to ask for the cloudberry committers or other people to help review and approve.

[tuhaihe]

You can see this action: https://github.com/tuhaihe/gpbackup/actions/runs/29551662660

Note:

• This is a case using the official release tarball of cloudberry-backup and cloudberry 2.1.0.
• Rocky 10 and Ubuntu 24.04 were not supported by Cloudberry 2.1, so these two cases failed.
• You can download the packages-linux-amd64 and packages-linux-arm64 after running this workflow to verify them.

cc: @leborchuk @woblerr PTAL.