Only set CUDA_PATH if it is not already set

[ptheywood]

%env CUDA_PATH=/usr/local/cuda

May set the CUDA verison to an incorrect version in some cases.

Plus this is non-portable if running on Windows.

Only set this, if it is not already set. Ideally set it relative to a CUDA version which is on the PATH already.

Alterantively we could modify FLAMEGPU2 to try finding nvcc on the path and getting the CUDA directory that way instead.

[ptheywood]

import os
if "CUDA_PATH" not in os.environ:
  import subprocess, pathlib
  cuda_path = str(pathlib.Path(subprocess.check_output(["which", "nvcc"]).decode("utf-8")).parents[1])
  %env CUDA_PATH="$cuda_path"

Seems to be a not terrible way to do this, though it relies on which being available still so is bash only. Unfortunately can't just use bash expansion in %env magic.