This reverts commit bac2bf6c45.
The problem was really that ensure-pip was giving us a
"ensure_pip_virtualenv_command" that didn't work on some systems.
Iaa3ecd05b64af6dd9b2ee17a39bcbe6cde8686ba fixes this underlying issue,
so we can revert to the original change that uses that to install the
tox environment.
Change-Id: I8ce9dceb721474d3220f6e72409481dc89875ee0
9c04709719