runBackground redirects stdout and stderr to files instead of command line

[lolgab]

It seems that the problem was introduced in #2377
In particular, the change was introduced in this commit: 1a43868

Before this PR stdout was redirected to mill's stdout, after it goes to os.pwd / "stdout.log" and stderr goes to os.pwd / "stderr.log"

[lihaoyi]

Probably going to call this a wontfix. The whole point of runBackground is you may have your terminal doing other things after, and so we can't just keep sending logs to the terminal after you call it. So to avoid just losing them, they have to go to log files, and people can tail the logs if necessary