authz-tcp doctest flake

[bleggett]

This was not-fixed by https://github.com/istio/istio.io/pull/15474#issuecomment-2258801446

(but the fact that we had a commented "make sure we nuke the env because preceeding tests aren't cleaning up" is suspicious)

The flake is random and is probably not due to authz-tcp tests themselves but to some preceding tests not properly cleaning up.