Fix untagged_image_cleanup workflow

[stuartleeks]

The untagged_images_cleanup workflow is failing with:

Checking for untagged versions for ci-devcontainer
jq: error (at <stdin>:1): Cannot index array with string "message"

E.g. https://github.com/devcontainers/ci/actions/runs/3802308704/jobs/6467665279#step:3:7