Don't modify the value of environment variables in test code

Remove any calls to

Environment.SetEnvironmentVariable

Environment variables by definition should be passed in from the environment and not altered while the process iss running.

I think we are currently getting away with this because the original values of the environment variables are only read once, and read early enough that they are captured before some test modifies them. But this is fragile.

For the tests that are doing this, we need to find some other way to satisfy what they need that doesn't involve changing the value of environment variables on the fly.