proposal: stdout by default

[adamgordonbell]

Use case

Earthly now outputs all output to stderr. Suggestion: move to stdout and reserve stderr for errors from Earthly. ( ie. failure to connect to buildkit)

Currently output can be moved to stdout using 2>&1 but it might make sense to default to this.

Another option would be a config flag, where the default for build logs can be changed.

Benefits

• play better with other CIs, like GHA which expect control messages on stdout ( see Add support for logging lines without target name prefix #3713 )
• following existing conventions

Alternative

Another option would be a config option, setting where the default for build logs can be changed. This would be more backwards compatible, though not clear if that is important in this case.