publish PR test images to GHCR

[turadg]

Problem

PRs have started failing more often like this,

Running test image for proposal upgrade-13
Unable to find image 'ghcr.io/agoric/agoric-3-proposals:test-upgrade-13' locally

In the same CI job that tag was built, so the runner is erroneously losing it.

Design

Try publishing the test images to GHCR in one job and running the tests in a separate job that pulls from GHCR.

It might be worthwhile to tackle use at the same time,

• build and push a 'use' image for each proposal #25

[turadg]

With #25 and #2 solved this is no longer motivated.