limit PIO CI to use v1.x branch of RF24Network #73

Job	Run time
check_formatting / cpp_linter	5s
build (adafruit_feather_rp2040) / build	40s
build (adafruit_itsybitsy_rp2040) / build	36s
build (adafruit_qtpy_rp2040) / build	22s
build (arduino_nano_rp2040_connect) / build	1s
build (pico) / build	1s
build (pimoroni_pga2040) / build	1s
build (pimoroni_picolipo_16mb) / build	1s
build (pimoroni_picolipo_4mb) / build	1s
build (pimoroni_tiny2040) / build	1s
build (sparkfun_micromod) / build	1s
build (sparkfun_promicro) / build	1s
build (sparkfun_thingplus) / build	1s
	1m 52s

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