ruby: improve failure message when missing cargoHash

Without this, doing e.g. `pkgs.mkRuby` without a `cargoHash` param would
mean that the `fetchCargoTarball` drv was not a FOD and thus had no
network access and would give confusing network failure messages. This
forces specifying non-null `cargoHash` if it is actually being used,
while allowing it to still be `null` when not needed.

pkgs/development/interpreters/ruby/default.nix

@@ -125,7 +125,7 @@ let
         cargoDeps = if yjitSupport then rustPlatform.fetchCargoTarball {
           inherit (finalAttrs) src;
           sourceRoot = "${finalAttrs.pname}-${version}/${finalAttrs.cargoRoot}";
-          hash = cargoHash;
+          hash = assert cargoHash != null; cargoHash;
         } else null;
 
         postUnpack = opString rubygemsSupport ''