ffmpeg: enable nvenc for x86 only
ffmpeg: enable nvenc for x86 only

It doesn't seem that cross builds are at fault but rather that
nv-codec-headers seems to be written for x86 and nothing else,
which is usually a good assumption as the nvidia drivers only
really exist for x86 (well, they do exist for ppc64le in the
CUDA toolkit but it's more effort to get that working and
tested)

[ci skip]

necrophcodr/void-packages,necrophcodr/void-packages,necrophcodr/void-packages,necrophcodr/void-packages,necrophcodr/void-packages