blender: use correct Python version

This partially reverts commit 46861969d7.
Using nixpkgs’s default Python was previously necessary to workaround a
problem that no longer exists: materialx always using the top-level
python3 instead of that which matches its dependents.

Using the same Python version as the upstream Blender release is desired
to ensure compatibility not only Blender itself but with the addon
ecosystem.

Also move the selection of non-nixpkgs-default Python to all-packages
rather than the pkg’s function args as that is the current preference
(as documented in
./pkgs/by-name/README.md#changing-implicit-attribute-defaults).

pkgs/top-level/all-packages.nix

@@ -29582,6 +29582,7 @@ with pkgs;
 
   blender = callPackage  ../applications/misc/blender {
     openexr = openexr_3;
+    python3Packages = python311Packages;
     inherit (darwin.apple_sdk.frameworks) Cocoa CoreGraphics ForceFeedback OpenAL OpenGL;
   };