boards: nrf: Update nrf54h and nrf54l yamls to use sysbuild in twister

Nrf54h's and nrf54l's targets are the first targets complex enough
that a sysbuild must be used on every build for them. This information
is reflected in their yamls. These entries tell twister to always use
sysbuild for those targets.

Signed-off-by: Maciej Perkowski <Maciej.Perkowski@nordicsemi.no>