[tests] Use net8.0 for the project used to restore workload manifest nuget, instead of net6.0 (#104067)

Author: radical

src/tasks/WorkloadBuildTasks/PackageInstaller.cs

@@ -123,7 +123,7 @@ private static string GenerateProject(IEnumerable<PackageReference> references)
             projectFileBuilder.Append(@"
 <Project Sdk=""Microsoft.NET.Sdk"">
     <PropertyGroup>
-        <TargetFramework>net6.0</TargetFramework>
+        <TargetFramework>net$(NETCoreAppMaximumVersion)</TargetFramework>
     </PropertyGroup>
     <ItemGroup>");