ci: Bring back ninja for dist builders

The primary reason for this is that make can result in a substantial
under utilization of parallelism, 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.

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