dumbhttp: use IPv6 if HGIPV6 is set to 1...
dumbhttp: use IPv6 if HGIPV6 is set to 1
This will fix flaky tests using dumbhttp.
The patch was tested on gcc112.fsffrance.org using the following command:
./run-tests.py -j 40 --runs-per-test 120 test-bundle2-remote-changegroup.t