Preview app example breaks if logs get too long

[markusthoemmes]

Currently, the example sets an env var with the respective log output to then reference that in the action to print the logs. Those env vars have a 128KB limit, which causes errors around Argument list too long. That's no good.

We should be able to refactor the example to not need env vars to sidestep that issue. We should probably also add a warning to the README to call that limit out.