nixos/lighthouse: use lib.getExe

        # make sure the chain data directory is created on first run

        mkdir -p ${cfg.beacon.dataDir}/${cfg.network}

        ${cfg.package}/bin/lighthouse beacon_node \

        ${lib.getExe cfg.package} beacon_node \

          --disable-upnp \

          ${lib.optionalString cfg.beacon.disableDepositContractSync "--disable-deposit-contract-sync"} \

          --port ${toString cfg.beacon.port} \