Fix Molecule CI validation failure for default scenario

[Copilot]

The molecule GitHub Actions job was failing before any scenario steps ran because the default scenario inherited Podman registry settings with unresolved environment variables. Under the CI-installed Molecule/Podman plugin stack, those became null and failed schema validation.

• Root cause

  • molecule/default/molecule.yml was effectively empty except for the verifier config.
  • The shared config in molecule/ext/molecule-src/molecule.yml defines platform registry settings via environment variables.
  • In CI, unset registry variables left platforms[*].registry.url as null, which the Podman driver schema rejects.

• Change

  • Override the inherited registry URLs in the default scenario so validation does not depend on optional registry environment variables.
  • Keep the scenario otherwise minimal and continue to inherit the shared Molecule configuration.

• Result

  • The default scenario now validates cleanly in CI and proceeds into the actual Molecule test sequence instead of failing during config/schema validation.

• Config override

  ---
  verifier:
    name: ansible
  platforms:
    - name: workspace-src-ubuntu_noble
      registry:
        url: ghcr.io
    - name: workspace-src-ubuntu_jammy
      registry:
        url: ghcr.io