fix(build): correct CUDAToolkit_ROOT path format for Windows #262

Job	Run time
linux-cpu-avx2-x86_64 build + test	31s
linux-vulkan-x86_64 build + test	1m 43s
linux-neon-arm64 build + test	43s
linux-opencl-x86_64 build + test	37s
linux-cpu-arm64 build + test	38s
windows-cpu-avx-vnni-x86_64 build + test	1m 37s
windows-cpu-x86_64 build + test	1m 50s
linux-opencl-arm64 build + test	1m 18s
linux-cpu-avx512-x86_64 build + test	33s
linux-cpu-x86_64 build + test	31s
android-x86_64 build + test	1m 54s
ios build	46s
isim build	43s
windows-gpu-x86_64 build + test	4m 21s
windows-cpu-avx2-x86_64 build + test	1m 57s
windows-openblas-x86_64 build + test	1m 35s
android-arm64-v8a build	34s
windows-vulkan-x86_64 build + test	1m 39s
macos build + test	46s
linux-openblas-x86_64 build + test	41s
linux-openblas-arm64 build + test	1m 35s
windows-opencl-x86_64 build + test	1m 39s
windows-cpu-avx512-x86_64 build + test	1m 44s
windows-llamafile-x86_64 build + test	1m 49s
linux-vulkan-arm64 build + test	2m 41s
release	0s
	34m 25s

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