Better way of customizing jobs in CI

The release procedure requires that we deploy_to_repo with DRY_RUN set first, and then, if no respin is required, deploy_to_repo without it so that the prepared files are copied over to our repo site. Gitlab either doesn't allow to re-run the same job with a different value for a previously set environment variable, or at least I can't find a way to do it.

Also, even though not required by the release procedure, I've accidentally confused some jobs and ran deploy_pkgs when trying to make deploy_to_repo work, and that also uploaded the packages to stork-testing Cloudsmith repo. When I had to upload the packages to stork by setting an environment variable, I was unable to do so.

What would be required, I think, is to move away from the current way of setting environment variables in jobs, and that looks something like this:

@slawek suggested using the parallel:matrix property among other solutions. https://mattermost.isc.org/isc/pl/pqa5sizky7bbbfzbyf3y8bbwiw