GHCR packages are created as private by default and the visibility
cannot be changed via the REST API. Reverting to docker.io/paritytech
until the package visibility is changed to public via GitHub UI at:
https://github.com/orgs/pezkuwichain/packages/container/ci-unified/settings
The mirror-ci-image.yml workflow has already populated GHCR - just
need to make it public, then update this file to use GHCR.
Now that the mirror-ci-image workflow has populated GHCR, switch
.github/env from docker.io/paritytech/ci-unified to our own
ghcr.io/pezkuwichain/ci-unified mirror.