[integration-tests] Fix etcd use of host.docker.internal on mac

[andrewmains12]

host.docker.internal doesn't work correctly for our etcd integration tests on Mac's, but localhost does. It used to be the case that host.docker.internal didn't resolve at all on Mac's--I encoded this fact into the health check logic for our docker container. Now it does resolve, so I've just switched to an explicit build constraint based version.