Fix ThinkNode M8 repeater/room server build: compile sensor helpers for all envs

[marcelverdult]

ThinkNode_M8_repeater and ThinkNode_M8_room_server currently fail to link on dev:

undefined reference to `EnvironmentSensorManager::querySensors(unsigned char, CayenneLPP&)'
undefined reference to `EnvironmentSensorManager::begin()'
undefined reference to `vtable for EnvironmentSensorManager'

variants/thinknode_m8/target.cpp instantiates EnvironmentSensorManager for every env, but EnvironmentSensorManager.cpp was only added to the companion envs' build_src_filter. This moves +<helpers/sensors> into the shared [ThinkNode_M8] section (same as ThinkNode M3/M6) and drops the now-redundant per-env entries.

Tested with pio run on dev (1934a49): ThinkNode_M8_repeater, _room_server, _companion_radio_ble, _companion_radio_usb, _kiss_modem all build. Without the change, _repeater fails with the errors above.