Since all bitcoin-ci-test images are now labeled, we can always
prune all dangling images, regardless of whether we are in
RESTART_CI_DOCKER_BEFORE_RUN.
To be safe, still prune all images if RESTART_CI_DOCKER_BEFORE_RUN
in case the filtering doesn't work, or if images were created on
an earlier version that did not assign labels.
This allows us or the user to perform batch operations on all
images produced by the ci, e.g. to prune all dangling images,
without affecting non-ci images.
