Harden BuildHomeBlocksDeps checkout ref resolution for stale/invalid branch inputs

[Copilot]

BuildHomeBlocksDeps was failing before build execution because actions/checkout was given a non-existent ref (stable/v6.x). This change makes ref selection resilient so workflow dispatches and callers don’t hard-fail on stale branch inputs.

• Ref resolution before checkout

  • Added a Resolve checkout ref step in .github/workflows/build_dependencies.yml (BuildHomeBlocksDeps).
  • Resolution logic:
    • use github.ref when inputs.branch is empty,
    • accept full refs (refs/...) directly,
    • for branch names, verify remote existence in the current repo,
    • fallback to github.ref with a warning when invalid/missing.

• Unified checkout behavior

  • Updated both checkout paths to consume steps.resolve-ref.outputs.checkout_ref:
    • Retrieve Code
    • Retrieve Recipe
  • Prevents early workflow failure when caller-provided inputs.branch is stale.

- name: Retrieve Code
  uses: actions/checkout@v4
  with:
    ref: ${{ steps.resolve-ref.outputs.checkout_ref }}