CI: upload build logs as artifacts for failed RPM builds

[thozza]

It seems that when we build RPMs in CI, we are instructing mock not keep any logs from the build. As a result, there is literally no information to help debugging a build failure.

We should be probably setting cleanup_on_failure to False when running mock and upload all build logs as artifacts.

Example of a build failure: https://gitlab.com/redhat/services/products/image-builder/ci/osbuild-composer/-/jobs/3223520188

CC @jrusz