Revert "ghcup-metadata: Drop output_name field"

This breaks the ghcup metadata generation on the nightly jobs.

To reproduce the failure:

nix shell -f .gitlab/rel_eng/ -c ghcup-metadata --pipeline-id=94361 --metadata=ghcup-0.0.7.yaml --date=2024-05-08 --version=9.11.20240506