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