Consider adding PHD runs with release propolis-server to CI

PHD runs in CI test using a debug propolis-server for its (theoretically) better debuggability when tests fail. By default, however, Omicron uses release builds of propolis-server, which can behave differently in surprising ways; see e.g. #292.

We should consider adding some kind of CI run that checks that release builds of the server pass our integration tests.