rure/update.sh: remove non portable nix hash invocation

As pointed out in #398598.

pkgs/by-name/ru/rure/update.sh

···
 {
   "pname": "$crate",
   "version": "$version",
-  "hash": "$(nix hash to-sri --type sha256 "$(printf '%s' "$prefetch" | head -n1)")"
+  "hash": "sha256:$(printf '%s' "$prefetch" | head -n1)"
 }
 EOF