[embedthumbnail] Set mtime correctly

Related: https://github.com/yt-dlp/yt-dlp/issues/67

yt_dlp/postprocessor/embedthumbnail.py

@@ -85,6 +85,8 @@ class EmbedThumbnailPP(FFmpegPostProcessor):
             thumbnail_filename = thumbnail_jpg_filename
             thumbnail_ext = 'jpg'
 
+        mtime = os.stat(encodeFilename(filename)).st_mtime
+
         success = True
         if info['ext'] == 'mp3':
             options = [
@@ -187,6 +189,8 @@ class EmbedThumbnailPP(FFmpegPostProcessor):
             os.remove(encodeFilename(filename))
             os.rename(encodeFilename(temp_filename), encodeFilename(filename))
 
+        self.try_utime(filename, mtime, mtime)
+
         files_to_delete = [thumbnail_filename]
         if self._already_have_thumbnail:
             info['__files_to_move'][original_thumbnail] = replace_extension(