Replace setx with set

[mwobensmith]

In various parts of the code, we try to be helpful when we detect a configuration that doesn't have the right environment variables set. Sometimes this is because Iris is being used as the moziris package, and other times it's because a user's machine has been running multiple copies of the project and Python caches what it thinks to be the current active code directory.

We display a message telling users how to reset two environment variables, and we detect platform to display the right instructions. However, for Windows, we are recommending the use of setx which is machine-level, whereas we need to instead suggest set which only affects the current session.

[mwobensmith]

Actually, now that we have #601 it is possible that we don't need this warning message at all. Let's remove the whole thing once the package work lands so we are assured that this won't be a problem.