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 are relevant).

PR-URL: nodejs/node#16298
Ref: nodejs/node#15754 (comment)
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>

doc/onboarding-extras.md

@@ -30,6 +30,7 @@
 | `test/*` | @nodejs/testing |
 | `tools/eslint`, `.eslintrc` | @not-an-aardvark, @silverwind, @trott |
 | async_hooks | @nodejs/async_hooks for bugs/reviews (+ @nodejs/diagnostics for API) |
+| build | @nodejs/build |
 | performance | @nodejs/performance |
 | platform specific | @nodejs/platform-{aix,arm,freebsd,macos,ppc,smartos,s390,windows} |
 | python code | @nodejs/python |