ci: fix Windows build

.github/workflows/windows-conda.yml

-name: Build Pinocchio via Conda
+name: Build Pinocchio for Windows via Conda
 on:
   pull_request:
   push:
@@ -54,6 +54,7 @@ jobs:
           -G "Visual Studio 16 2019" -T "ClangCl" -DCMAKE_GENERATOR_PLATFORM=x64 ^
           -DCMAKE_INSTALL_PREFIX=%CONDA_PREFIX%\Library ^
           -DCMAKE_BUILD_TYPE=Release ^
+          -DPINOCCHIO_SITELIB_ROOT=%CONDA_PREFIX% ^
           -DPYTHON_EXECUTABLE=%CONDA_PREFIX%\python.exe ^
           -DBUILD_WITH_URDF_SUPPORT=ON -DBUILD_PYTHON_INTERFACE=ON ^
           ..