multimedia/vmaf: unbreak built-in models after 1868dedecc9a

$ ffmpeg -i foo.y4m -i bar.y4m -lavfi libvmaf -f null -
[...]
libvmaf WARNING no such built-in model: "vmaf_v0.6.1"
[Parsed_libvmaf_0 @ 0x8061b33c0] could not load libvmaf model with version: vmaf_v0.6.1
[AVFilterGraph @ 0x8061b6880] Error initializing filter 'libvmaf'

(cherry picked from commit 355579c2ebff93c22fe8d3bb1004103c4e2154b7)