Adjust macOS CI to use three simultaneous jobs

[belegdol]

As of December 2024, all the used gitlab hosted runners have 7 GB of RAM and 14 GB of storage. macOS runner has 3 CPUs whereas Linux and Windows runners have 2. Since Windows and Linux jobs already use 3 jobs, it should also work for macOS.

[belegdol]

macOS job only got two minutes faster which could just as well be the normal variance. Looks like this might not be worth it.

[cuavas]

I don’t want to do this, because on a Linux VM with 6GB of RAM and lots of things stopped to free up memory (e.g. graphical session manager stopped), a full build hits swap at a couple of points even with -j4 due to the way it ends up trying to build the memory system templates in libemu and the Lua bindings templates in libfrontend at the same time, and the M68000 CPU core files. I want to stick with -j2 on these 7GB VMs to ensure there’s some headroom. I’ve explained this every time someone opens a PR to do this.