gitlab-ci tag for jobs requiering ipv6

a job in arti needs to be able to bind ::1 to succeed. This works on some runners, but not others. Every tpa runner has ipv6 (or so I've heard), so tagging the job that way fixes the issue. Having something more explicit, and which can be used for non-tpa runners with ipv6, would be nice to have. Possibly in the great future where ipv4 is dying and almost everything is ipv6, having the opposite tag (this runner supports ipv4) could be nice to have too.