`om ci run` stdout is no longer just a list of store paths

[srid]

If your flake (like this one) defines custom steps which run commands that writes to stdout, then om ci run's own stdout too would inherit them.

This suggests we can no longer rely on om ci run's stdout to contain specific data.

One way forward is to replace --json (#270) with --write-results-to=FILE (writes result in json to that file), and pass that around to next steps (which incidentally facilitates the likes of #203).