Use new GH container registry for autotest images

[bziemons]

Motivation

GitHub changed their registry URL and it is to be expected that the old URL wont work forever.

Modification

The new registry URL is ghcr.io[1] and that needs to be used in the Actions workflows and the build_images.py.

[bziemons]

Postponed until ghcr is out of beta and the GITHUB_TOKEN gets the required permissions:

If you want to authenticate to GitHub Container Registry in a GitHub Actions workflow, then you must use a personal access token (PAT). The GITHUB_TOKEN does not currently have the required permissions. During the GitHub Container Registry beta, the only supported form of authentication is the PAT.

https://docs.github.com/en/free-pro-team@latest/packages/guides/using-github-packages-with-github-actions#authenticating-to-github-container-registry