Output envoy logs only in the case of failures

In [e2e tests](https://github.com/envoyproxy/xds-relay/pull/62), in order to declutter the logs, we should output envoy logs only in case of failures.

One idea is to generate [artifacts](https://help.github.com/en/actions/configuring-and-managing-workflows/persisting-workflow-data-using-artifacts) via github actions and attach them to the workflow run.

Start with the e2e tests referenced in the issue and the GitHub Actions workflow that runs them. Determine how Envoy logs are currently collected, then verify that logs are retained or surfaced for failed runs without cluttering successful runs.

Written by the indexing model from the issue text.