Fix external downloader availability when using --ffmpeg-location (#12318)

This fix is only applicable to the CLI option

Authored by: Kenshin9977

yt_dlp/postprocessor/ffmpeg.py

@@ -202,7 +202,7 @@ class FFmpegPostProcessor(PostProcessor):
 
     @property
     def available(self):
-        return self.basename is not None
+        return bool(self._ffmpeg_location.get()) or self.basename is not None
 
     @property
     def executable(self):