Start with a fresh venv each time in jenkins

The venv can get dirty between runs, it's best to start with a clean
one when in jenkins.

This will eventually need to be namespaced so that we can have multiple
runs going at the same time, or rely on some other segregation to keep
multiple runs from operating on the same paths at the same time. This is
the least impactful change we can make to improve our jenkins stability.