python: disable network on builders by default
    
    This is the safe default we should have since beginning, that's btw. why
    web-UI (frontend side) and copr-cli has the default to False since ever.
    
    Discussed with Packit folks, originally thought it is a Packit docs
    issue, related links:
    https://github.com/packit/packit.dev/pull/484
    https://github.com/packit/packit/issues/1386