diffoscope: fix on aarch64-darwin

This was comparing stdenv.hostPlatform (an attr set) to a string. This
comparison always returned not-equal, which resulted in trying to
evaluate the broken gnumeric on macOS. This change fixes that to compare
the "system" string as was intended originally.

pkgs/tools/misc/diffoscope/default.nix

@@ -219,7 +219,7 @@ python3.pkgs.buildPythonApplication rec {
     # oggvideotools is broken on Darwin, please put it back when it will be fixed?
     ++ lib.optionals stdenv.isLinux [ oggvideotools ]
     # This doesn't work on aarch64-darwin
-    ++ lib.optionals (stdenv.hostPlatform != "aarch64-darwin") [ gnumeric ]
+    ++ lib.optionals (stdenv.hostPlatform.system != "aarch64-darwin") [ gnumeric ]
   ));
 
   nativeCheckInputs = with python3.pkgs; [