ci: dynamically match makejobs with cores

Previously jobs were running on a large multi-core server where 10 jobs
as default made sense (or may even have been on the low side).

Using hosted runners with fixed (and lower) numbers of vCPUs we should
adapt compilation to match the number of cpus we have dynamically.

This is cross-platform compatible with macos and linux only.

ci/test/00_setup_env.sh

@@ -35,7 +35,7 @@ fi
 
 echo "Fallback to default values in env (if not yet set)"
 # The number of parallel jobs to pass down to make and test_runner.py
-export MAKEJOBS=${MAKEJOBS:--j4}
+export MAKEJOBS=${MAKEJOBS:--j$(if command -v nproc > /dev/null 2>&1; then nproc; else sysctl -n hw.logicalcpu; fi)}
 # Whether to prefer BusyBox over GNU utilities
 export USE_BUSY_BOX=${USE_BUSY_BOX:-false}