Reduce environment pollution

[Tyriar]

Testing #226

This feature results in a lot of added environment variables:

We should try clean these up whenever we can. If any activate script is called, can we remove all the activate variables?

[karthiknadig]

@Tyriar Are you saying remove it after activation?

[Tyriar]

@karthiknadig yes, but I have a more radical idea in #241

[Tyriar]

## Extension: ms-python.vscode-python-envs

Environment variables for shell activation

- `VSCODE_PYTHON_PWSH_ACTIVATE=& d:\playground\py-test\.venv\Scripts\Activate.ps1`
- `VSCODE_PYTHON_BASH_ACTIVATE=source /d/playground/py-test/.venv/Scripts/activate`
- `VSCODE_PYTHON_CMD_ACTIVATE=d:\playground\py-test\.venv\Scripts\activate.bat`

Actual env in pwsh:

VSCODE_PYTHON_AUTOACTIVATE_GUARD
VSCODE_PYTHON_BASH_ACTIVATE
VSCODE_PYTHON_CMD_ACTIVATE