Anything involving a PSDrive, including ENV:, will only affect the current process. But - and I’ll explain my process here, in case that helps you - I Googled “.NET Environment Class.” That got me to https://msdn.microsoft.com/en-us/library/system.environment(v=vs.110).aspx, which is the class you were using to set a variable.
I ran into an ‘issue’ / limitation while using this method. I needed to create an environment variable for the local app-data directory %USERPROFILE%\AppData\Local\Apps\2.0</code>. I tried creating a variable which used the $env:USERPROFILE for the target parameter, to access user profiles on remote machines. as a work around I used the following code to access the local app-data!