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

This fix is only applicable to the CLI option

Authored by: Kenshin9977

yt_dlp/downloader/external.py

@@ -457,8 +457,6 @@ class FFmpegFD(ExternalFD):
 
     @classmethod
     def available(cls, path=None):
-        # TODO: Fix path for ffmpeg
-        # Fixme: This may be wrong when --ffmpeg-location is used
         return FFmpegPostProcessor().available
 
     def on_process_started(self, proc, stdin):

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):