Added MSBuild conditional to nuget.config

Author: Joshua-Lester3

nuget.config

@@ -7,7 +7,7 @@
         <add key="EssentialCSharp" value="https://pkgs.dev.azure.com/intelliTect/_packaging/EssentialCSharp/nuget/v3/index.json" />
     </packageSources>
     <disabledPackageSources>
-        <add key="EssentialCSharp" value="false" />
+        <add key="EssentialCSharp" value="!$(AccessToNugetFeed)" />
     </disabledPackageSources>
     <packageSourceMapping>
         <packageSource key="nuget">