python3Packages.lightgbm: fix lib_lightgbm.so location

cmake installs lib_lightgbm.so into the wrong place,
`$out/lib/python3.13/site-packages/lib/lib_lightgbm.so`,
which is not part of the locations `lightgbm/libpath.py` considers.

Instead of patching that file, which seems to have regressed, set the
`__BUILD_FOR_PYTHON` `cmakeBool` (as in `pyproject.toml`), which causes
the .so file to be placed to a location that will be considered for
dlopen()'ing later, removing the need for any patching alltogether.

The new location of the .so file is also more aligned with how other
python packages in nixpkgs do it,
`$out/lib/python3.13/site-packages/lightgbm/lib/lib_lightgbm.so`.

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

@@ -55,18 +55,6 @@ buildPythonPackage rec {
     hash = "sha256-yxxZcg61aTicC6dNFPUjUbVzr0ifIwAyocnzFPi6t/4=";
   };
 
-  # Patch dll search path to fix proper discovery of lib_lightgbm.so
-  #   Exception: Cannot find lightgbm library file in following paths:
-  # /nix/store/...-python3.13-lightgbm-4.6.0/lib/python3.13/site-packages/lib_lightgbm.so
-  # /nix/store/...-python3.13-lightgbm-4.6.0/lib/python3.13/site-packages/lightgbm/bin/lib_lightgbm.so
-  # /nix/store/...-python3.13-lightgbm-4.6.0/lib/python3.13/site-packages/lightgbm/lib/lib_lightgbm.so
-  postPatch = ''
-    substituteInPlace lightgbm/libpath.py \
-      --replace-fail \
-        'curr_path.parents[0] / "lib",' \
-        'curr_path.parents[1] / "lib",'
-  '';
-
   build-system = [
     scikit-build-core
   ];
@@ -102,6 +90,9 @@ buildPythonPackage rec {
   cmakeFlags = [
     (lib.cmakeBool "USE_GPU" gpuSupport)
     (lib.cmakeBool "USE_CUDA" cudaSupport)
+    # Set in pyproject.toml for `cmake.args` in `[tool.scikit-build]`,
+    # but not set by our hooks.
+    (lib.cmakeBool "__BUILD_FOR_PYTHON" true)
   ];
 
   optional-dependencies = {