Make sure journalctl -b is properly captured on shutdown

[ogayot]

With recent changes, the journalctl -b command would fail to run:

      File "subiquity/server/controllers/shutdown.py", line 118, in copy_logs_to_target
        await self.app.command_runner.run(
      File "subiquity/server/runner.py", line 98, in run
        proc = await self.start(cmd, **opts)
                     ^^^^^^^^^^^^^^^^^^^^^^^
    TypeError: DryRunCommandRunner.start() got an unexpected keyword argument 'stdout'

Fixed by making sure that:

• the output of journalctl -b is captured instead of going into the journal (which in itself feels like a bug).
• the command runner expects the stdout, stderr and other possible keyword arguments.

[ogayot]

With this change, the installer-journal.txt file will contain some boilerplate data as shown in the example above:

Running as unit: run-u1060.service
not running: journalctl -b      <---- This is the output of the command
Finished with result: success
Main processes terminated with: code=exited/status=0
Service runtime: 4ms
CPU time consumed: 3ms

If this isn't ok, we should use the --quiet option from systemd-run. This should remove the boilerplate. Otherwise, we can use another mechanism that does not lean on systemd-run.