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.
how do these get cleaned up? why not just rm -rf and create a new one in its place?

I had code to do this, but rejected it because it adds to build time.  If it is more reliable, perhaps it makes sense to put it back?