Runtime 5.15-25.08: Consider using ffmpeg from runtime

Freedesktop runtime **25.08** (and related runtimes GNOME **49**, KDE **6.10**, and KDE **5.15-25.08**) provides FFMPEG.

The build manifest of ffmpeg from the Freedesktop runtime version 25.08, which the related runtimes inherit, is here:

https://gitlab.com/freedesktop-sdk/freedesktop-sdk/-/blob/freedesktop-sdk-25.08.6/elements/components/ffmpeg.bst#L22

Using it from runtime might reduce ffmpeg-related maintenance, but there might be some config differences related to the supported codecs.

Note: This project may be incompatible with the version provided by the runtime.

Thanks.