lib: fix event race condition with -e

Commit c5b07d4e ("lib: fix beforeExit not working with -e") runs the to-be-evaluated code at a later time than before because it switches from process.nextTick() to setImmediate().

It affects -e 'process.on("message", ...)' because there is now a larger time gap between startup and attaching the event listener, increasing the chances of missing early messages. I'm reasonably sure process.nextTick() was also susceptible to that, only less pronounced.

Avoid the problem altogether by evaluating the code synchronously. Harmonizes the logic with Module.runMain() from lib/module.js which also calls process._tickCallback() afterwards.