[ffmpeg] Fix embedding subtitles (#9063)

Changed command line parameters for ffmpeg when embedding subtitles.
Changed to ‘-map 0:v -c:v copy -map 0:a -c:a copy’

youtube_dl/postprocessor/ffmpeg.py

@@ -363,8 +363,10 @@ def run(self, information):
         input_files = [filename] + sub_filenames
 
         opts = [
-            '-map', '0',
-            '-c', 'copy',
+            '-map', '0:v',
+            '-c:v', 'copy',
+            '-map', '0:a',
+            '-c:a', 'copy',
             # Don't copy the existing subtitles, we may be running the
             # postprocessor a second time
             '-map', '-0:s',