Update instructions to use pipx?

[jcormier]

In Ubuntu 24.04, they have made it pretty hard to install python packages without a virtualenv.

To help with this, Ubuntu includes pipx by default, which helps auto managing the venvs for you.
pipx — Install and Run Python Applications in Isolated Environments

• Create a python virtual environment

$ pipx install snagboot[gui]
  installed package snagboot 2.3, installed using Python 3.12.3
  These apps are now globally available
    - snagfactory
    - snagflash
    - snagrecover
done! ✨ 🌟 ✨

• Check that snagboot has been installed and you have access to snagboot commands

$ snagrecover -h
$ snagflash -h
$ snagfactory -h