When Cookbook installs vllm via `pip install --user vllm`, pip pulls in
nvidia-cuda-* wheels under /app/.local but doesn't set CUDA_HOME or
create /usr/local/cuda. vllm 0.22+ then crashes during engine init:
RuntimeError: Could not find nvcc and default cuda_home='/usr/local/cuda' doesn't exist
After that, the mixed cuda-nvcc 13.3 / cuda-runtime 13.0 wheel combo
fails FlashInfer's JIT sampler with:
error: "CUDA compiler and CUDA toolkit headers are incompatible"
Detect the pip-installed nvcc on startup, point CUDA_HOME at it, and
default VLLM_USE_FLASHINFER_SAMPLER=0 (sampler only, no attention
impact) so the engine boots. No-op when vllm isn't installed.
Fixes#214.
Co-authored-by: sirs <sirs@local>
