setup: install pip in virtualenv to make sure we have the latest version

Older versions are good enough for bootstrapping (and might also be good for
everything else) but gives:

WARNING: You are using pip version 19.3.1; however, version 20.0.2 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.

The user might still get this warning initially, but then it goes away ...