FindCUDA: Revert "Use find_program instead of find_path to find nvcc"

Revert commit 067ee518 (FindCUDA: Use find_program instead of
find_path to find nvcc, 2019-10-31).  It causes `CUDA_TOOLKIT_ROOT_DIR`
to contain the path to nvcc rather than the toolkit directory.
Status Job ID Name Coverage
  External
passed ghostflow-check-master