fix eval

cc @zimbatm

(https://nix-cache.s3.amazonaws.com/log/iyn9cwk0kymi4mzva0dzpqwnzm3cg4a4-nixpkgs-tarball-18.03pre118395.891c3721ed.drv)

pkgs/development/compilers/rust/cargo.nix

@@ -56,6 +56,6 @@ rustPlatform.buildRustPackage rec {
     description = "Downloads your Rust project's dependencies and builds your project";
     maintainers = with maintainers; [ wizeman retrry ];
     license = [ licenses.mit licenses.asl20 ];
-    platforms = platforms.linux ++ platforms.darwin;
+    platforms = [ "x86_64-linux" "x86_64-darwin" ];
   };
 }