chore(tvix/nix-compat-derive): Remove references to nixrs

Some references to nixrs had sneaked in while writing the original
CL so this just removes those.

Change-Id: I635adbd2de46c7c1f31e1d449396253a78e1b762
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12727
Reviewed-by: Vladimir Kryachko <v.kryachko@gmail.com>
Autosubmit: Brian Olsen <me@griff.name>
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/nix-compat-derive/src/internal/mod.rs

@@ -113,7 +113,7 @@ impl<'a> Container<'a> {
                 Data::Enum(variants)
             }
             syn::Data::Union(u) => {
-                ctx.error_spanned(u.union_token, "Union not supported by nixrs");
+                ctx.error_spanned(u.union_token, "Union not supported by nix-compat");
                 return None;
             }
         };