Fix reset interactive build cancellation

Do not Build.Cancel if build has already completed.

src/Interactive/EditorFeatures/Core/Extensibility/Interactive/ResetInteractive.cs

@@ -83,7 +83,7 @@ internal Task Execute(IInteractiveWindow interactiveWindow, string title)
 
             // If the user hits the cancel button on the wait indicator, then we want to stop the
             // build.
-            waitContext.CancellationToken.Register(() =>
+            var buildCancellation = waitContext.CancellationToken.Register(() =>
                 CancelBuildProject(), useSynchronizationContext: true);
 
             // First, start a build.
@@ -94,6 +94,8 @@ internal Task Execute(IInteractiveWindow interactiveWindow, string title)
                 return;
             }
 
+            buildCancellation.Dispose();
+
             // Then reset the REPL
             waitContext.Message = InteractiveEditorFeaturesResources.ResettingInteractive;
             await interactiveWindow.Operations.ResetAsync(initialize: true).ConfigureAwait(true);