nixos/libretranslate: fix stdout logging
Currently, the libretranslate service does not appear to output any
logs to the systemd journal, even though it prints log lines to stdout
when run outside of a systemd service. This commit fixes the problem by
setting PYTHONUNBUFFERED to disable Python's stdout buffering.
(cherry picked from commit 98a1f4602c40c3b8af79ae7c1c198db3c15ffc0a)