Runtime 5.15-25.08: Consider using ffmpeg from runtime

[yakushabb]

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.