fix(build): add CUDAToolkit_ROOT environment variable for windows-gpu build

Author: Gioee

.github/workflows/main.yml

@@ -391,6 +391,8 @@ jobs:
           make build/llama.cpp.stamp ${{ matrix.make }} LLAMA_ARGS="--target llama"
           make build/llama.cpp.stamp ${{ matrix.make }} LLAMA_ARGS="--target ggml"
           make build/llama.cpp.stamp ${{ matrix.make }} LLAMA_ARGS="--target ggml-base"
+        env:
+          CUDAToolkit_ROOT: "C:\Program Files\NVIDIA GPU Computing Toolkit"
 
       - name: copy backend modules to dist
         if: false #matrix.name == 'windows-gpu'