Fix so pip install runs in venv

[ErikBjare]

Right now it sometimes tries to install Python packages, but fails since it doesn't install them in the Python environment it is running in (I assume).

Could probably set some env variables for this, like PYTHONHOME or PYTHONPATH.

[darwinssy]

bash: PYTHONPATH: command not found help me sir

[ErikBjare]

Although this might mess up the gptme installation.

We could add a method to use either, since it could be useful to self-update and install extras.

Might be important to look over which Python is used by the REPL and shell, and ensure it is the same. And write some tests for it.

@Leonardo2801 Sorry, still working on this.