Adjust run.cmd to prepend python to PATH instead of append (#42)

This fix allows run.cmd on windows to successfully find mozmill-env's version of python, rather than the system's. In cases on windows where there was no python installed, it wasn't a problem; but on end-user systems with python installed, mozmill-environment testrun scripts were finding the system's version of python and so would fail.

Neither linux or macos were affected, their path order was fine.