Disable Github CI for draft PRs with label

[jasnell]

@nodejs/build @nodejs/github-bot ... It would be helpful if the automatic GitHub CI could be disabled for Work-in-progress draft PR's with a "skip-ci" label. This is good for PRs that we want to open early to get awarenes/feedback but that we know aren't ready for testing yet.

[phillipj]

Would it make sense to not trigger CI on draft PRs in general? Not having to apply a label at all sounds convenient from a distance..