Switch to Node 12 for CI
Switch to Node 12 for CI

While we updated Cesium to work on 12, we never actually made CI use it.
Traditionally we always use the active LTS for CI (which covers 12 until
October when it becomes 14).

likangning93/cesium,CesiumGS/cesium,CesiumGS/cesium,progsung/cesium,likangning93/cesium,likangning93/cesium,CesiumGS/cesium,CesiumGS/cesium,progsung/cesium,AnalyticalGraphicsInc/cesium,CesiumGS/cesium,likangning93/cesium,likangning93/cesium,AnalyticalGraphicsInc/cesium