[FFmpegMerger] Fix merger discard input timestamps

Some website save video-only and audio-only file with non-zero
timestamp. Therefore, when merging them, ffmpeg should keep original
timestamp.

According to ffmpeg doc: https://ffmpeg.org/ffmpeg.html

`-copyts`: Do not process input timestamps, but keep their values
without trying to sanitize them. In particular, do not remove the
initial start time offset value.

yt_dlp/postprocessor/ffmpeg.py

@@ -822,7 +822,7 @@ class FFmpegMergerPP(FFmpegPostProcessor):
     def run(self, info):
         filename = info['filepath']
         temp_filename = prepend_extension(filename, 'temp')
-        args = ['-c', 'copy']
+        args = ['-c', 'copy', '-copyts']
         audio_streams = 0
         for (i, fmt) in enumerate(info['requested_formats']):
             if fmt.get('acodec') != 'none':