Bumping Mercurial to 5.7.1
Since the `Dockerfile.released-hg` does not specify the version in its pip install of `hg-evolve`, we'll get the latest released version of evolve in new builds.
Since the `Dockerfile.released-hg` does not specify the version in its pip install of `hg-evolve`, we'll get the latest released version of evolve in new builds.