I got frustrated by the lack of useful output when services failed to
start in CI, so I thought I'd mess around.

I made some changes so that:

1. we wait for services with health checks to report as healthy,
2. we do not rebuild the engine or custom connector when they start,
3. the health checks work (apart from ndc-postgres, which will come
later), and
4. we use environment variables rather than command-line arguments where
possible.

I have also renamed the "agent" binary to "custom-connector", matching
its crate, because it was driving me a little crazy.

V3_GIT_ORIGIN_REV_ID: 8d672b0b25438b54d47368ce82cd236cfdd4e554