Add -dotnet=ver command arg to Flax.Build to specify .NET SDK version to use for build

Source/ThirdParty/nethost/nethost.Build.cs

@@ -32,7 +32,7 @@ public class nethost : ThirdPartyModule
         // Get .NET SDK runtime host
         var dotnetSdk = DotNetSdk.Instance;
         if (!dotnetSdk.IsValid)
-            throw new Exception($"Missing NET SDK {DotNetSdk.MinimumVersion}.");
+            throw new DotNetSdk.MissingException();
         if (!dotnetSdk.GetHostRuntime(options.Platform.Target, options.Architecture, out var hostRuntime))
         {
             if (options.Target.IsPreBuilt)