Add a CI run for no-IPv6

[A5rocks]

I'm not sure this is possible. I think there's a file we can write to on Linux, but IDK if GitHub Actions gives us that ability.

If it isn't possible this issue should be closed.

[A5rocks]

sudo sysctl net.ipv6.conf.all.disable_ipv6=1 doesn't outright error but it doesn't seem to effect anything. I reverted #3136 and CI still passed: https://github.com/A5rocks/trio/actions/runs/13587103299/job/37984466705.