Don't show 'Executing...' line for subsequent @runs_once invokes

[bitprophet]

Jacob Kaplan-Moss noticed that using @runs_once still causes an Executing task line to print out for every call of the decorated task, even though (as designed) the body of the task is only being executed the first time.

This is ugly and feels like a bug -- while the task will return its memoized value, it's not actually running/executing, and so it's just extra noise when used as part of a multi-host run.