python3Packages.lightgbm: override stdenv if built with cuda support

From https://github.com/NixOS/nixpkgs/pull/507972#discussion_r3053161889:

> [This prevents] the other stdenv and its cc from leaking into the
> build. The latter is somewhat optional, but the leakage can result in,
> on one hand, in mixing object files from different cc releases ("works
> in practice but nobody offers guarantees"), and on the other in the
> hypothetical possibility that cuda tools use up wrong cc.
>
> If lightgbm builds cuda kernel, it "should" conditionally override
> stdenv, so far we don't have a better system to configure this
> transparently to individual packages...

pkgs/development/python-modules/lightgbm/default.nix

@@ -42,7 +42,10 @@
 assert gpuSupport -> !cudaSupport;
 assert cudaSupport -> !gpuSupport;
 
-buildPythonPackage (finalAttrs: {
+let
+  effectiveStdenv = if cudaSupport then cudaPackages.backendStdenv else stdenv;
+in
+buildPythonPackage.override { stdenv = effectiveStdenv; } (finalAttrs: {
   inherit (pkgs.lightgbm)
     pname
     version