logger: Don't set SRT_LOG_TO_JOURNAL if there are other log destinations
SRT_LOG_TO_JOURNAL=1
makes utilities from the steam-runtime-tools
suite output their diagnostic messages to only the systemd Journal,
but if we have explicitly wrapped a subprocess tree with srt-logger
,
we likely expect to be sending output from that subprocess tree to a
log file and/or the terminal.
steamrt/tasks#466