Your problem is spaces around =.
This will work (attention to space before closing quote):
Console.WriteLine(Environment.GetEnvironmentVariable("ASPNETCORE_ENVIRONMENT "));
The space after ASPNETCORE_ENVIRONMENT in this code is not a typo! The problem in the question was having extra space (in SET…), so you must use the same space in GetEnvironmentVariable().
As noted by Isantipov in a comment, an even better solution is to remove the spaces entirely from the SET command:
set ASPNETCORE_ENVIRONMENT=Production