The cause of this failure is that num-modular 0.6.6 has switched to version 0.5 of num-bigint, this probablly should have come with a semver bump upstream, but it didn't.
num-bigint 0.5 in turn no longer supports rand 0.8. I've prepared patches in debcargo-conf which fix the package to use num-modular 0.6.6, num-bigint 0.5 and rand 0.10 but I haven't uploaded them because of their potentially large API impact, and the fact that num-prime is currently not in testing and not used by any applications in debian. I have also posted links to the fixes upstream at https://github.com/uutils/num-prime/issues/120
