CI: make 070-build.bats use safe image names

In preparation for maybe some day being able to run build tests
in parallel.

SUPER IMPORTANT NOTE! BUILD TESTS CANNOT BE PARALLELIZED YET!
buildah, when run in parallel, barfs with:

    race: parallel builds: copying...committing...creating... layer not known

Until this is fixed, podman-build can never be run in parallel.
See containers/buildah#5674

This PR is simply cleaning things up so, if/when that day comes,
the ensuing parallelize PR will be short & sweet.

Signed-off-by: Ed Santiago <[email protected]>