Build automation streamlined: use "abuild-common" accounts

[jimklimov]

Following up from #6827 (introducing accounts for general automated builders) and #6824 (adding Jenkinsfile for oi-userland PRs, assuming some environment preparation), we should eat our dogfood after accepting and merging #6827 and update the preparation script to use "abuild" group instead of "builders" (and probably update the build server for PRs with that), and document "abuild-common" package as an explicit requirement for any local setup of Jenkins to build oi-userland so that the expected account is defined on such system.