Reduce number of threads in ./build_rmp.sh make

At the moment the pod cta-build seems to get stuck while building CTA in a dev machine.

[...]
-- Configuring done (0.6s)
-- Generating done (0.2s)
-- Build files have been written to: /shared/CTA/build_rpm/RPM/BUILD/cta-5-dev/build
+ cmake --build . -j16
[0/467] Building CXX object frontend/grpc/CMakeFiles/cta-frontend-async-grpc.dir/RequestMessage.cpp.o

By looking with htop I see that all 16 cores are being used at 100% and the pod gets stuck. This is probably not a good idea, considering that we are building CTA inside a container, which is bounded by several constraints.

I suggest that we always leave some cores free.