authz/github: allow list group repos 404 to persist to cache
Created by: bobheadxi
Potential solution to https://github.com/sourcegraph/customer/issues/742 . Looks like GitHub has some odd behaviours related to teams called owners
etc. that may cause "list repo" on an existing repo to return 404. Retrying on a 404 will not make it un-404, so we allow these errors to persist to cache. I've added a docstring inline to explain why this is safe.
Test plan
Existing behaviour should hold (tests should pass)