chore: use PYTHONUNBUFFERED to output logs instantly

What does this merge request do and why?

Even with feat: enable Python faulthandler to debug segfa... (!3657 - merged), the segfaults still happen silently: https://cloudlogging.app.goo.gl/tUzUxcu5opmguBJBA

Related issue: https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/issues/1589+

Let's set PYTHONUNBUFFERED according to https://cloud.google.com/run/docs/tips/python:

To see unbuffered logs from your Python application, set the environment variable PYTHONUNBUFFERED. When you set this variable, stdout and stderr data is immediately visible in the container logs, instead of being held in a buffer until a certain amount of data has accumulated or the stream is closed.