d/rules: ensure $(NJOBS) propagates down into cmake sub-builds
When running cmake -j$(NJOBS) in debian-full-build, it will spawn various additional cmake builds. However, since it doesn't pass the -j arg, those sub-builds will use whatever the default is. This results in my build OOM'ing on a machine with 16 cores and only 32gb of memory.
This ensures that those sub-builds honor NJOBS.