knot2 fails to build in hydra, due to IFD in knot2 package #4634

Hydra (and by extension nixpkgs) doesn't allow import from derivation, which prevents building knot2 in these environments.

To reproduce:

nix eval .#knot-rs --option allow-import-from-derivation 0

I believe this is due to the use of runCommand in rust-src.nix. Possibly this can be replaced with just nixpkg's sources lib stuff.

Related: #436