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)