ci: Bring back ninja for dist builders The primary reason for this is that make can result in a substantial under utilization of parallelism (noticed while testing on a workstation), mostly due to the submake structure preventing good dependency tracking and scheduling. In f758c7b2a78 (Debian 6 doesn't have ninja, so use make for the dist builds) llvm.ninja was disabled due to lack of distro package. This is no longer the case with the CentOS 7 base, so bring ninja back for a performance boost.