chore: Migrate RL Scanner to usable action

[tanya732]

Changes

• Replaced the local .github/actions/rl-scanner composite action with the shared rl-scan@main action in the existing rl-scanner.yml reusable workflow.
• Wired the newly required secrets, SIGNAL_HANDLER_DOMAIN and PRODSEC_PYTHON_TOOLS_REPO, through release.yml and rl-scanner.yml.
• Fixed the artifact path ($(pwd) → ${{ github.workspace }}) so the scanner can locate the built .tgz artifact.
• Removed the obsolete local composite action.