See the patch is merged, if there are other issues lets open a ticket specific on the issue

Is the issue causing failures still exists?

Patch to install packages via puppet as this is affecting newly
installed slaves and causing unpredictable job failures:
https://gerrit.ovirt.org/65291

we don`t mean moving the whole job (this cannot be node easilty because there is some crucial stuff needed for engine install missing inside mock, such as systemd), just moving the build part of it to run inside mock_runner.sh and use the same scripts the rest of the world uses to build engine.

If you mean move the job entirely into standard ci and mock, if I remember correctly there was some limitation on running the upgrade-scripts inside mock, though I'm not familiar enough with the job details. I have an open ticket https://ovirt-jira.atlassian.net/browse/OST-3 to create a new upgrade suite inside ovirt-system-tests using Lago, but I don't ETA on that yet.
- you remember what was the limitation on running the upgrade suites inside mock?

BTW - as a side note - as far as I can tell, we have a limited number of jobs that don't run in standard-ci, a path might worth exploring is having a separate labels & hostgroups for those slaves and applying the jenkins_slave packages on them only. That way 95% of our slaves will be 'thin', and another 5% for legacy jobs, which should disappear eventually.