Build options broken with sysbuild

[carlescufi]

After investigating #79057, we have discovered that build options, both with west and ninja, are broken with sysbuild.

So, this works:
west build -b nrf54h20dk/nrf54h20/cpuapp --build-opt=-j1 samples/hello_world (built by a single core)
but this is broken:
west build -b nrf54h20dk/nrf54h20/cpuapp --build-opt=-j1 --sysbuild samples/hello_world (ignores the-j1, uses all cores)

The only way to limit the cores with sysbuild is:
CMAKE_BUILD_PARALLEL_LEVEL=1 west build -b nrf54h20dk/nrf54h20/cpuapp --sysbuild samples/hello_world