When doing local builds, we get phase reporting lines in the log file,
they look like '@nix {"action":"setPhase","phase":"unpackPhase"}'.
With the ssh-ng protocol, we do have access to these messages, but since we
are only including messages of type resBuildLogLine in the logs, the phase
information does not end up in the log file.
The phase reporting could probably be improved altoghether (it looks like it
is kind of accidental that these JSON messages for phase reporting show up
but others don't, just because they are actually emitted by nixpkgs' stdenv),
but as a first step I propose to make ssh-ng behave in the same way as local builds do.
