ssh-ng: also store build logs to make them accessible by `nix log`

Right now when building a derivation remotely via

    $ nix build -j0 -f . hello -L --builders 'ssh://builder'

it's possible later to read through the entire build-log by running
`nix log -f . hello`. This isn't possible however when using `ssh-ng`
rather than `ssh`.

The reason for that is that there are two different ways to transfer
logs in Nix through e.g. an SSH tunnel (that are used by `ssh`/`ssh-ng`
respectively):

* `ssh://` receives its logs from the fd pointing to `builderOut`. This
  is directly passed to the "log-sink" (and to the logger on each `\n`),
  hence `nix log` works here.
* `ssh-ng://` however expects JSON-like messages (i.e. `@nix {log data
  in here}`) and passes it directly to the logger without doing anything
  with the `logSink`. However it's certainly possible to extract
  log-lines from this format as these have their own message-type in the
  JSON payload (i.e. `resBuildLogLine`).

  This is basically what I changed in this patch: if the code-path for
  `builderOut` is not reached and a `logSink` is initialized, the
  message was successfully processed by the JSON logger (i.e. it's in
  the expected format) and the line is of the expected type (i.e.
  `resBuildLogLine`), the line will be written to the log-sink as well.

Closes #5079

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