Fix Warning CS8600: Converting null literal or possible null value to non-nullable type.

[jpobst]

In #103 this line was added:

string ev = Environment.GetEnvironmentVariable (envVar);

With NRT, Environment.GetEnvironmentVariable returns string? instead of string, causing the warning:

Warning CS8600: Converting null literal or possible null value to non-nullable type.

We can avoid it by using var instead of string.