Start with a fresh venv each time

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

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.
Apparently we DIDN'T configure rebuilding the workspace. So these won't get cleaned up. Not sure where it's appropriate to add this code.