python3.pkgs.setuptools-rust: adjust setupHook inclusion condition (#480005)

  # integrate the setup hook to set up the build environment for cross compilation

  # this hook is automatically propagated to consumers using setuptools-rust as build-system

  #

  # Only include the setup hook if python.pythonOnTargetForTarget is not empty.

  # No need for the setup hook when python.pythonOnTargetForTarget is empty,

  # or when the host & target platforms are the same.

  #

  # python.pythonOnTargetForTarget is not always available, for example in

  # pkgsLLVM.python3.pythonOnTargetForTarget. cross build with pkgsLLVM should not be affected.

  setupHook =

    if python.pythonOnTargetForTarget == { } then

    if

      python.pythonOnTargetForTarget == { }

      || (lib.systems.equals stdenv.hostPlatform stdenv.targetPlatform)

    then

      null

    else

      replaceVars ./setuptools-rust-hook.sh {