subprocess.run() with preexec_fn forces Python to use fork() instead
of posix_spawn(). In Frigate's main process (75+ threads), fork()
creates a child that inherits locked mutexes from other threads. The
child may deadlocks e.g. on a pysqlite3 mutex before it can exec()
ffmpeg.
Replace preexec_fn=lower_priority (which calls os.nice(19)) with
prefixing the ffmpeg command with "nice -n 19", achieving the same
priority reduction without requiring preexec_fn. This allows Python
to use posix_spawn() which is safe in multithreaded processes.
Fixes both the primary export path and the CPU fallback retry path.
