Fix cancelling a debug task

`CancelDebugExecution` was supposed to exit early when the runspace was
unusable. Basically forgot the `!` in the `if` statement.

Author: SeeminglyScience

src/PowerShellEditorServices/Services/PowerShell/Execution/SynchronousPowerShellTask.cs

@@ -382,7 +382,7 @@ internal void MaybeAddToHistory()
 
         private void CancelDebugExecution()
         {
-            if (_pwsh.Runspace.RunspaceStateInfo.IsUsable())
+            if (!_pwsh.Runspace.RunspaceStateInfo.IsUsable())
             {
                 return;
             }