[Downstream CI] Fix build job limt

[pearzt]

As it turns out, CMake counterintuitively only respects CMAKE_BUILD_PARALLEL_LEVEL when not passing --parallel.

[jplehr]

So this change still makes the build run in parallel?
That's indeed counter intuitive.

[pearzt]

So this change still makes the build run in parallel?

Yes, since CMAKE_BUILD_PARALLEL_LEVEL is set. Passing --parallel makes it use Ninja's default number of processes (104, in our case), indepedent of CMAKE_BUILD_PARALLEL_LEVEL.

https://cmake.org/cmake/help/latest/manual/cmake.1.html#cmdoption-cmake-build-j