message: ci: only run IPv6 tests on the IPv6 machines
  Changing our v6 CI build to be v6 only. Ideally it'll be a little faster since the v6 resources are scarce.
  Risk Level: Low
  Testing: is there a way to test CI changes?
  Docs Changes: n/a
  Release Notes: n/a
diff: @@ -10,6 +10,8 @@ export ENVOY_BUILD_DIR=/tmp/envoy-docker
  export TEST_TYPE="bazel.ipv6_tests"
  +export BAZEL_EXTRA_TEST_OPTIONS="--test_env=ENVOY_IP_TEST_VERSIONS=v6only"
  +
  function finish {
  echo "disk space at end of build:"
  df -h
  