It seems that, at some point, the ability to run tox on old servers (with old
setuptools) got broken - and it was only working on our Jenkins instance by
dint of reusing the tox environments.

Let's try to get tox to do the right thing, and remove the guff from
jenkins/prepare_synapse.sh.

(There is a separate question about whether the jenkins builds should be using
tox to prepare the virtualenv at all here, but that is somewhat orthogonal).