Update Arcade SDK

Update to a newer version of Arcade to fix the build.

Author: martincostello

NuGet.config

@@ -2,6 +2,7 @@
 <configuration>
   <packageSources>
     <add key="NuGet" value="https://api.nuget.org/v3/index.json" />
+    <add key="dotnet-eng" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-eng/nuget/v3/index.json" />
     <add key="dotnet-tools" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-tools/nuget/v3/index.json" />
     <add key="dotnet-core" value="https://dotnetfeed.blob.core.windows.net/dotnet-core/index.json" />
     <add key="azureadwebstacknightly" value="https://www.myget.org/F/azureadwebstacknightly/api/v3/index.json" />

global.json

@@ -4,7 +4,7 @@
   },
 
   "msbuild-sdks": {
-    "Microsoft.DotNet.Arcade.Sdk": "5.0.0-beta.20180.5",
-    "Microsoft.DotNet.Helix.Sdk": "5.0.0-beta.20180.5"
+    "Microsoft.DotNet.Arcade.Sdk": "5.0.0-beta.20431.1",
+    "Microsoft.DotNet.Helix.Sdk": "5.0.0-beta.20431.1"
   }
 }