Fix topaz template install using a custom template, which does not use ghcr.io.

• Extract the policy registry from the resource passed in, instead of using the hardcoded https://ghcr.io
• Fallback to use ghcr.io when the extraction fails

With this change, existing templates continue to work, all template tests pass, and no updates are required.

The complete fix requires adding a policy registry field, which will require updating all existing templates as well.