bug#43418: [PATCH] gnu: youtube-dl: Refer to ffmpeg.

[Mark H Weaver]

Hi Tobias,

> Addresses <https://issues.guix.gnu.org/43418>.
>
> * gnu/packages/video.scm (youtube-dl)[inputs]: Add ffmpeg.
> [arguments]: Fall back to input ffmpeg instead of searching $PATH.

Looks good to me.  Thank you!

     Mark