fix(worker-init): declare a non-root OCI image user #2072

Planned implementation PR

Set the nvcf_worker_init OCI image user to a numeric non-root UID/GID. Keep the worker-init and trust bundle installer executables accessible, and add a built-image test that reads Config.User from the image metadata.

Parent: #2071

Done when

• Image metadata declares a non-root user on all supported architectures.
• Image content and entrypoint tests pass.
• The worker-init unit tests pass.