[Xamarin.Android.Tools.AndroidSdk] Fix CS8600 in AndroidSdkBase (#107)

In commit b2d9fdf, this line was added:
string ev = Environment.GetEnvironmentVariable (envVar);
With C#8 Nullable Reference Types,
`Environment.GetEnvironmentVariable()` returns `string?`, not `string`.
This causes the warning:
Warning CS8600: Converting null literal or possible null value to non-nullable type.
We can avoid the CS8600 by using `var` instead of `string`.

Author: jpobst

src/Xamarin.Android.Tools.AndroidSdk/Sdks/AndroidSdkBase.cs

@@ -138,7 +138,7 @@ public virtual void Initialize (string? androidSdkPath = null, string? androidNd
 protectedIEnumerable<string>GetSdkFromEnvironmentVariables()
 {
 foreach(stringenvVarinAndroidSdkEnvVars){
-stringev=Environment.GetEnvironmentVariable(envVar);
+varev=Environment.GetEnvironmentVariable(envVar);
 if(String.IsNullOrEmpty(ev)){
 continue;
 }