Unset VIRTUAL_ENV env var: pgzero mode fails, otherwise.

[tmontes]

CONTEXT:

• Found failure after setting up dev env with:

  $ python3.8 -m venv venv
  $ ./venv/bin/activate
  (venv) $ pip install -e ".[dev]"
  (venv) $ mu-editor

• In this scenario, the VIRTUAL_ENV var is set, when running Mu.

• For some not-100% clear reason, this leads Mu to use the dev venv Python binary to run PGZero code, instead of it's own venv where pgzero is actually installed.

SELF-NOTE:

• The root cause was not found.
• This is more of a quick-fix. 🤷🏻‍♂️😅

[tmontes]

CONFIRMED:

• Built Wheel.
• Created fresh 3.8 venv.
• Launched mu-editor
• PGZero mode works.

ALSO:

• PGZero mode works in the dev env setup described above.

[tmontes]

NOTE:

• Somewhat like with PR #2465, four Run tests / Test PiOS ... checks failed.
• Every other one is ok.

MERGING