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.