limit threads for spicy build processes during Zeek package installation

[mmguero]

Found when trying to compile the icsnpp-hart-ip parser for the Raspberry Pi "minihog" installation, things would eventually get into swap and then run out of memory, never completing.

After some experimentation I determined that the best solution was to set HILTI_JIT_PARALLELISM, which by default is set to the number of processors. I've set it to half that number, but it can be overridden with the BUILD_JOBS variable as used in some of the GitHub build actions.

This resolved the issue of the raspberry pi hedgehog build for me.