Pre-load NVIDIA shared libraries on Linux

Mirrors the Windows preload block from #1775. When onnxruntime-gpu is
installed via pip with nvidia-cudnn-cu12, the .so files sit under
venv/lib/pythonX.Y/site-packages/nvidia/<pkg>/lib/ and the dynamic
linker never sees them. LD_LIBRARY_PATH cannot be set after Python
starts.

Pre-loads every lib*.so* via ctypes.CDLL with RTLD_GLOBAL before
onnxruntime opens its CUDA provider. Also extends LD_LIBRARY_PATH so
child processes (ffmpeg) inherit the path.

Fixes "libcudnn.so.9: cannot open shared object file" on pip-only
Linux installs.