CUDA: Fix NVCC executable search during compiler detection
The result variable was CUDAToolkit_NVCC_EXECUTABLE instead of _CUDA_NVCC_EXECUTABLE, which the rest of the code used.
I noticed this code can actually be simplified to a single get_filename_component() call, so I did that too.
Fixes: #20847 (closed)
Backport: release
Edited by Brad King