fix: do not try to upgrade images that have not been built
This fixes an error on jenkins:
2021-03-31 02:49:58,709 Unable to find image 'logilab/cubicweb:py35-stretch-3.27.0' locally
docker: Error response from daemon: manifest for logilab/cubicweb:py35-stretch-3.27.0 not found: manifest unknown: manifest unknown.
See 'docker run --help'.
Traceback (most recent call last):
File "build.py", line 357, in <module>
debian_dists=["stretch", "buster"], images=images, rebuild=args.checkrebuild
File "build.py", line 289, in build
assert (out.returncode, out.stdout) == (0, b""), out
This occurs if the image build failed but the script still try to update the packages in the (missing) image.
This MR adds a test to upgrade only if the image was built correctly in the previous step.