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/484https://github.com/packit/packit/issues/1386
Merge: #2215