* ci: split unstable build and test workflows
In order to make multi-arch builds the buildx action will use the
docker-container driver, which does not support loading images, and thus
the jvb image won't be built based on the "base-java" image, in turn based
on the "base" image. That works only when pushing.
If we setup buildx to use the "docker" driver it will just build for the
current architecture, but it will properly load images and the base
images will be used correctly.
One downside is that jobs cannot be run in parallel, but it seems to be Fast Enough (R) for now.
Saúl Ibarra Corretgé