Handle CUDA_PATH correctly

[gfardell]

TomographicImaging/CIL#1596

I seem to recall a way to set environment variables for an enviroment inside the meta.yaml.

Failing that we should use scripts in activate.d and deactivated.d

[gfardell]

conda env config vars set CUDA_PATH="X"

might work. But I'm not sure that the cudatoolit package is actually installing the cudart libraries on windows.

[gfardell]

For context this is on a windows installation of the tigre conda package. This appears to be handled with linux.

[gfardell]

https://conda.io/projects/conda/en/latest/user-guide/tasks/manage-environments.html#setting-environment-variables

[gfardell]

This is an issue with tigre trying to find the cudart library rather than relying on the application.

CERN/TIGRE#579