As described in
https://alt.fedoraproject.org/pub/alt/virtio-win/latest/deprecated-README
the virtio-win ISOs have moved to a new location with a different layout,
which broke the various scripts in tools/.

At this point, manually fetching the latest ISO, and copying the directory
structure manually to spice-nsis/drivers/ should be enough for the
nsis script. Make sure you remove the .pdb files, they contain debug
information and can be quite large, no need to ship them in the installer