Change I316e9587b6e290cd421b47f506c91dbebe0975c0 had a rather
embarrasing oversight in that it copied the /usr/bindep-env/bin/pip
invocation for upgrading pip to all the other venv's.
i.e. we were upgrading hte bindep-env pip over and over, and not
actually the pip in the working venv. The os-testr install on older
platforms has now broken because it still tries to install with the
ancient inbuilt pip -- local testing has confirmed that it works with
the updated pip.
Change-Id: I22c549b5f9b9e3882fcd2340946d2850b0b2f86b
efbb9b8961