Automatically retry e2e on release branches; else soft fail

Created by: nicksnyder

This is an alternative to disabling e2e tests outright, as proposed in https://github.com/sourcegraph/sourcegraph/pull/6163.

e2e test will still run on all branches so we can continue collecting data, but failures will not block merging. Unfortunately, we still have to wait for tests to complete before getting a green status so there is no "builds are faster" win here.

In the cases where we do want to block on e2e tests (e.g. renovate branches, release branches), failures will be automatically retried twice.