Using dkms to install hw-specific kernel modules, I get:
/usr/bin/dkms: line 864: /usr/lib/linux-kbuild-${$kernelver%.*}/scripts/sign-file: bad substitution
This is a fault in dkms itself, (or theoretically in bash).
It affects dkms 3.0.5-1 and 3.0.5-2. Downgrading to dkms 3.0.3 removes this problem.
Works for me, 3.0.5-2 at least. Right now upgraded nvidia-dkms 515.57-1 => 515.65.01-1 and then downgraded back because 32-bit wine games stopped working (UPD: it was delayed lib32-nvidia-utils update, now everything's good). But dkms itself worked smoothly both ways.