fix: disable LBT phase-2 optimisation for loramac-node sx127x backend

[seagull9000]

The T-Beam v1.x is still a useful device in the MeshCore ecosystem and a ZephCore build is viable with some supporting PRs.

This is one such PR.

The loramac-node sx127x backend holds a modem mutex for the full duration of async RX. lora_config() returns -EBUSY if called while that mutex is held, so the phase-2 path (skip hwCancelReceive, let send_async handle RX→TX transition) leaves the mutex permanently locked and silently drops all TX.

Disable LBT for backends that set _loramac_node = true so RX is always cancelled before attempting to configure for TX.

Note:

_loramac_node is a bool member of LoRaRadioBase, which is the shared base class for all radio adapters. It defaults to false in the base constructor (line 33). Only one radio adapter overrides it:

// SX127xRadio.cpp — constructor
_loramac_node = true;

Every other adapter — SX126xRadio, LR1110Radio, LR2021Radio — never touches it, so they all stay
false.