Have one orchestrator workflow that trigger all the other builder workflows

[joyeecheung]

Currently the builder workflows are split, so that when one of the platform fails, others can still trigger the merge workflow and keep the nightly tag for that platform updated. They are triggered nightly individually.

It would be better to have a main nightly orchestrator workflow that triggers them instead, this would allow an external repository to trigger that orchestrator workflow on an event (for example, when V8 is updated in the main repository) instead of having to trigger the platform-specific workflows individually.