Suggestion: scrape the top comment of a PR for side-tag: and use that for target

Reference:
- Centos reading zuul.message: https://gitlab.com/redhat/centos-stream/ci-cd/zuul/jobs/-/blob/a82c29327d0e8d0f26e7825768cfbce4c9ccb625/playbooks/pre-mock-build.yaml#L14-24
- Job to patch roles/fedpkg-build-ng: https://pagure.io/zuul-distro-jobs/blob/master/f/roles/fedpkg-build-ng/tasks/main.yaml#_27-31

Alternative:
- Redefine target var in fedora-zuul-jobs-config. Probably not a good idea to bloat that repo with implementations, especially since the approach here seems to be very generalizable to all callers