gnome2.GConf: fix build by pinning python312

python3 (python313) is missing "2to3":

  $ nix-build -A gnome2.GConf
  this derivation will be built:
    /nix/store/hmvb4pp7kna92hx0fs42nilqbq81jqyb-gconf-3.2.6.drv
  building '/nix/store/hmvb4pp7kna92hx0fs42nilqbq81jqyb-gconf-3.2.6.drv'...
  Running phase: unpackPhase
  unpacking source archive /nix/store/rf8vbdgdsbv05vnn0cbwcvpqck45b3jk-GConf-3.2.6.tar.xz
  source root is GConf-3.2.6
  setting SOURCE_DATE_EPOCH to timestamp 1358820019 of file "GConf-3.2.6/ChangeLog"
  Running phase: patchPhase
  /nix/store/nlqc1vm5psq2jjr5df6z25gkxkfqbvxx-stdenv-linux/setup: line 269: 2to3: command not found

pkgs/desktops/gnome-2/platform/GConf/default.nix

@@ -8,7 +8,7 @@
   ORBit2,
   libxml2,
   polkit,
-  python3,
+  python312,
   intltool,
 }:
 
@@ -46,7 +46,7 @@ stdenv.mkDerivation rec {
   nativeBuildInputs = [
     pkg-config
     intltool
-    python3
+    python312
     glib
   ];