CI: Add Rocky8 workflow with test matrix support

This commit introduces a new GitHub Actions workflow for building and
testing Apache Cloudberry on Rocky Linux 8, enabling automated builds,
RPM packaging, and regresssion testing alongside the existing Rocky 9
and Ubuntu 22.04 pipelines.

Triggers:
- Push to main branch
- Pull requests modifying this workflow file
- Scheduled: Every Monday at 02:00 UTC
- Manual workflow dispatch with optional test selection

Author: tuhaihe