formats.xml: fix cross compilation

`python3.pkgs` does not splice, so with a package-set setup to
cross-compile, we end up building xmltodict for the host platform. By
using `python3Packages.xmltodict` instead, we only use xmltodict for the
build platform, which is all that is needed for generating XML documents
at build-time.

(cherry picked from commit ae98bf9246)

pkgs/pkgs-lib/formats.nix

@@ -898,14 +898,13 @@ rec {
           pkgs.callPackage (
             {
               runCommand,
-              python3,
               libxml2Python,
+              python3Packages,
             }:
             runCommand name
               {
                 nativeBuildInputs = [
-                  python3
-                  python3.pkgs.xmltodict
+                  python3Packages.xmltodict
                   libxml2Python
                 ];
                 value = builtins.toJSON value;