Crux of the issue was that we were checking whether a venv activation
script exists on local filesystem, which is obviously wrong for remote
projects. This PR also does away with `source` for venv activation in
favor of `.`, which is compliant with `sh`

Co-authored-by: Lukas Wirth <lukas@zed.dev>

Closes #34648

Release Notes:

- Python: fixed activation of virtual environments in terminals for
remote projects

Co-authored-by: Lukas Wirth <lukas@zed.dev>