doc: add @nodejs/build to onboarding-extras.md
Make sure contributors know to cc @nodejs/build when submitting issues that are build related (in addition to whatever platform is relevant).
Ref: https://github.com/nodejs/node/pull/15754#issuecomment-337622625
Checklist
-
documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc