CUDA: improve regex for CUDA Toolkit root from nvcc verbose output

The original regular expression was greedy and would match any
environment variable ending with `TOP` (like `DESKTOP`). This is an
issue on windows where `nvcc -v` would output all environment variables
before the compiler's verbose output.

To resolve this issue we use a tighter match algorithm that looks
for `#$ TOP=` instead of `TOP=`.

Fixes: #22158