packaging: allow running packaging with custom uid+gid for CentOS rpmbuild in CentOS 7 has a bug causing rpmbuild to fail with "Bad owner/group" if spec or source files are owned by a different user: https://github.com/rpm-software-management/rpm/issues/2 This makes it very annoying to try and build the CentOS RPMs on CentOS with Docker. As an alternative, this change makes it possible to do so, using an environment variable. Differential Revision: https://phab.mercurial-scm.org/D5571