yt-dlp's ffmpeg_location does not reach its own partial-download check

Fact

--ffmpeg-location (option ffmpeg_location) is honoured by yt-dlp's postprocessors, but not by every internal availability check. The guard for section downloads — --download-sections, or download_ranges in the API — calls FFmpegFD.available() with no downloader attached, so the probe it performs can only search PATH.

With ffmpeg installed somewhere off PATH and pointed at correctly, conversion and tagging work while a trimmed download dies on:

ERROR: You have requested downloading the video partially,
but ffmpeg is not installed. Aborting

The message is wrong twice over: ffmpeg is installed, and yt-dlp has been told where it is.

Why it matters

The error names the one thing the operator already handled, so the search goes to the ffmpeg install and the flag's spelling rather than to the lookup path. It bites hardest where ffmpeg deliberately lives outside PATH — a vendored binary, a per-project virtualenv, a locked-down image.

How to apply

Prepend the directory to PATH in the process rather than relying on the option alone; the option can stay for the postprocessors.

os.environ["PATH"] = os.pathsep.join([str(ffmpeg_dir), os.environ.get("PATH", "")])

The binary must be named ffmpeg — yt-dlp matches on basename, so a versioned filename needs a symlink. Note also that ffprobe is a separate lookup: cover-art embedding needs it and raises a fatal postprocessing error without it, even when ffmpeg itself is found.