[Sponsorblock] minor fixes (#2793)

* preserve mtime - Closes #2769
* keep concat spec on failure

Authored by: nihil-admirari

yt_dlp/postprocessor/ffmpeg.py

@@ -384,11 +384,9 @@ def concat_files(self, in_files, out_file, concat_opts=None):
 
         out_flags = list(self.stream_copy_opts(ext=determine_ext(out_file)))
 
-        try:
         self.real_run_ffmpeg(
             [(concat_file, ['-hide_banner', '-nostdin', '-f', 'concat', '-safe', '0'])],
             [(out_file, out_flags)])
-        finally:
         os.remove(concat_file)
 
     @classmethod

yt_dlp/postprocessor/modify_chapters.py

@@ -68,9 +68,11 @@ def remove_chapters(file, is_sub):
         # Renaming should only happen after all files are processed
         files_to_remove = []
         for in_file, out_file in in_out_files:
+            mtime = os.stat(in_file).st_mtime
             uncut_file = prepend_extension(in_file, 'uncut')
             os.replace(in_file, uncut_file)
             os.replace(out_file, in_file)
+            self.try_utime(in_file, mtime, mtime)
             files_to_remove.append(uncut_file)
 
         return files_to_remove, info