Fixing the backwards compat test to use the stage2 with backwards compatible runtimes.

test/dotnet-back-compat.Tests/GivenThatWeWantToBeBackwardsCompatibleWith1xProjects.cs

@@ -42,7 +42,7 @@ namespace Microsoft.DotNet.Cli.Build.Tests
             var outputDll = testInstance.Root.GetDirectory("bin", configuration, target)
                 .GetFile($"{testAppName}.dll");
 
-            new DotnetCommand()
+            new DotnetCommand(DotnetUnderTest.WithBackwardsCompatibleRuntimes)
                 .ExecuteWithCapturedOutput(outputDll.FullName)
                 .Should().Pass()
                 .And.HaveStdOutContaining("Hello World");