`VenvWrapper` requires setting `with_pip=True` to work correctly

[vpratz]

By default, VenvWrapper (via EnvBuilder) does not include pip. As the pip in the path is then the one of the host environment, the pip install call will be executed in the wrong environment. Setting use_pip=True resolves this, but I think many (like I did) would assume this to be the default. Maybe we can change the default for the wrapper.