New pip policy in Bookworm

[robertjensen]

Raspbian Bookworm has a new policy for handling pip that makes the current bootstrap script fail.

I believe we should adapt to the policy described here:
https://www.raspberrypi.com/documentation/computers/os.html#using-pip-with-virtual-environments

As far as I can tell, this means that bootstrap should be changed to run
python -m venv --system-site-packages ~/.env

And the update of .bashrc should have this line added:
source ~/.env/bin/activate