python3.pkgs.setuptools-rust: adjust setupHook inclusion condition

Commit e656303867 (PR #467817) made
`setuptools-rust` work automatically in cross compilation scenarios by
replacing `setuptoolsRustBuildHook` with `setuptools-rust.setupHook`.
However it broke builds such as:

```sh
nix build -Lf. pkgsCross.armv7l-hf-multiplatform.buildPackages.python3.pkgs.cryptography
```

As reported here:

https://github.com/NixOS/nixpkgs/pull/467817#issuecomment-3731773060

This adjustment fixes the above build, while also maintaining the
following working:

```sh
nix build -Lf. pkgsCross.armv7l-hf-multiplatform.python3.pkgs.libcst
```

pkgs/development/python-modules/setuptools-rust/default.nix

@@ -43,11 +43,16 @@ buildPythonPackage rec {
   # 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 {