Fix permission issues in embed/merge

[mikiher]

This fixes #3146.

This changes the way the output of the embedding/merging tools is written back to the original directory, so that the permissions of the original audio files are kept intact.

Before this, we used fs.move(), which moved the ffmpeg output to the original file, effectively replacing the original permissions with whatever permissions the ffmpeg process assigned to its output file. Instead, we now pipe the contents of the ffmpeg output into the original file, so the contents of the original file are replaced but all file attributes (e.g. permissions) remain intact.

[advplyr]

Thanks!