Have bench use and manage different virtualenvs?

[N-Wouda]

We currently rely on sys.executable and pip to install/uninstall PyVRP. It might be nicer if bench manages its own virtualenvs for different PyVRP versions. Don't know how much work this'd be, and I also don't know if it's useful because we already get a bit of this by putting bench itself inside a virtualenv, possibly managed by a package manager.