[elastic-agent]: ensure that the enroll command also logs to file if configured

[simitt]

The enroll command can be automatically triggered when starting the docker container. In which case the LOGS_PATH should be respected.

Code path that needs to be changed:
https://github.com/elastic/beats/blob/master/x-pack/elastic-agent/pkg/agent/cmd/enroll.go#L197-L200

[blakerouse]

This is because of this change here: #24466

I think the change is correct, I would not expect the enroll command by default to write to a log file. We could change the container subcommand when it spawns the enroll command to send the stdout and stderr to a file when LOGS_PATH is set.

Actually with LOGS_PATH set I would expect all output from the container command to go to a log file. So we could set that up so it writes both to stdout and to a logfile. The logfile would need to be seperate from the main elastic-agent.log as they should not collide. Maybe elastic-agent-startup.log?

[simitt]

That sounds very reasonable to me.