ipv6 broken on github actions

Several tests, such as ``distributed/cli/tests/test_dask_scheduler.py::test_defaults``, fail on github actions CI on the ubuntu-latest image. Forcing the ``has_ipv6()`` function to return False makes the problem disappear.
Cannot reproduce locally.

This is a follow-up from #4504. I left in the PR:
- in ``.github/workflows/ci.yaml`` and ``distributed/utils_test.py``, a switch ``export DISABLE_IPV6=1``
- a new workflow which demonstrates the problem with a single faulty test, ``.github/workflows/ipv6_debug.yaml`` (normally disabled)