Update GH Actions Workflow cicd.yml to bust EWTS Docker cache when remote ref changes

[cmaynard-ngwpc]

• Improved CI/CD handling of EWTS builds
  • Updated GitHub Actions workflow to resolve EWTS_REF to its commit SHA and pass it as the EWTS_CACHE_BUST build arg.
  • Ensures the EWTS Docker layer rebuilds when a branch/tag HEAD changes, while preserving cache stability when using fixed commit hashes.

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows project standards (link if applicable)
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future todos are captured in comments
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
• Reviewers requested with the Reviewers tool ➡️