Consider publishing all external test images to Github packages

[smlambert]

In the past, we included the image generation during the test pipeline because we were supporting an extremely large number of variations / tags.

Since we have trimmed back the number of variants we support, we could consider building and publishing these images to ghcr.io when material inside of the external directory changes, and not during every test pipeline run.

This issue is to investigate the number of images we would want to publish (perhaps all of sanity.external ones that get run during EA pipelines), look at creating a workflow that builds and publishes them, and updates in our test pipeline code to look for an image and pull from the ghcr.io registry if its present, otherwise fall back to building the image from scratch versus always building from scratch (determine how much execution time is saved, the ease or complexity of such a change).

Once we gather some of the above information, we can make a decision on right next steps.

Helpful doc for pushing to ghcr.io from a workflow.

Related: #5657 (comment)