Be able to force the backend temporarily

[smarie]

We can imagine that a user working on some opensource project and cloning it will maybe have different settings than the project's continuous integration engine.

It could therefore be quite interesting to allow users to override the venv backend through a commandline or a global option.

This would be even interesting if combined to a "no_venv" choice of backend, in order to directly work on a passthrough (see #301 )