[ci] Set PYTHONUNBUFFERED=1 on the CI yaml level to avoid cluttered output on docker builds (see bob/bob.devtools#2)

This ensure that if the database was not cleaned on last run, it
won't block waiting for the answer to the question.

No database cleanup usually means that the tests were forcefully
stopped.

beat.web is a website, it doesn't belong to pypi.

Currently the environment is "pre-created" on the runner
machine.

This separates more cleanly the goal of each part. At the same
time the prefix path file can be kept in the sources rather than
being in /tmp that could be altered during a run.

This ensure that whatever the outcome of the tests, the prefix is
cleaned up. Currently these are piling up in the /var/tmp folder.