The Supreme Court is, at least for now, siding with a federal employee whom President Trump is trying to fire. Yet in a case that could shed light on whether Mr. Trump will be able to scale back the size of the federal workforce, the high court’s action essentially kicks the can down the road — if only until Wednesday, when a district judge is slated to hear arguments in the case. It denies Mr. Trump a quick win but holds out the possibility of a victory.