tests/bsim: sysbuild: Set primary MCU idx properly

When using sysbuild,
set the primary MCU index to be the app core
only when we are building the test app in the app core.

Otherwise, if the test app is build in the net core,
the test command line parameters won't be sent by default
to it, and tests will fail.

Luckily, these tests are only built in CI using sysbuild
when we target the 5340bsim app core, and in all but 2,
even if sysbuild was used, this setting was propagated
to the zephyr build if we had separate net and app core
images.
The issue was introduced during the transition to
hwmv2.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>