Add missing 'using System' for TimeSpan in NetCoreProjectTestCase.cs

Agent-Logs-Url: https://github.com/NuGet/NuGet.Client/sessions/4c4dad41-1936-4936-a55a-05ff5849d166

Co-authored-by: nkolev92 <[email protected]>

Author: Copilot

test/NuGet.Tests.Apex/NuGet.Tests.Apex/NuGetEndToEndTests/NetCoreProjectTestCase.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.IO;