Improve Windows guidance for CUDA override environment variables

[wunianze666-netizen]

Summary

• make cextension.py generate platform-aware set/clear instructions for override environment variables
• replace Linux-only export/unset guidance in CUDA/ROCm override errors and warnings with Windows-specific instructions when running on Windows
• add regression tests for the new instruction helpers

Why

The current override guidance is shown to Windows users as well, but it only suggests Unix shell commands. This change keeps the existing Unix guidance while making the same messages actionable on Windows.

Validation

• ran a local stub-based regression script to verify the Windows instruction strings and a Windows-targeted CUDA override error path
• ran python -m py_compile bitsandbytes/cextension.py tests/test_cuda_setup_evaluator.py

[matthewdouglas]

Thanks, but this looks like a duplicate of #1957.