CUDA: Capture all nvcc output when extracting toolkit root

Since commit fb2afef6 (CUDA: Support nvcc symlinking to ccache,
2021-01-07) and commit 3cef91a3 (CUDA: Always extract CUDA Toolkit
root from nvcc verbose output, 2021-02-03) we always run the command
`nvcc -v __cmake_determine_cuda` to look for the toolkit root in its
stderr.  On Windows, that command may print to stdout instead, so
capture that as well.