Change BALENA_DEVICE_TYPE to VARIANT in start-miner.sh

[shawaj]

Currently we are using both BALENA_DEVICE_TYPE and VARIANT to determine which config to load:

hm-miner/start-miner.sh

Lines 12 to 21 in 9564845

	if [ "$BALENA_DEVICE_TYPE" = "rockpi-4b-rk3399" ]; then
	cp /opt/miner/docker.config.rockpi "/opt/miner/releases/$HELIUM_GA_RELEASE/sys.config"
	OVERRIDE_CONFIG_URL="${ROCKPI_MINER_CONFIG_URL:-https://helium-assets.nebra.com/docker.config.rockpi}"
	elif [ "$BALENA_DEVICE_TYPE" = "intel-nuc" ]; then
	cp /opt/miner/docker.config.5g "/opt/miner/releases/$HELIUM_GA_RELEASE/sys.config"
	OVERRIDE_CONFIG_URL="${5G_MINER_CONFIG_URL:-https://helium-assets.nebra.com/docker.config.5g}"
	elif [ "$VARIANT" = "COMP-PISCESP100" ]; then
	cp /opt/miner/docker.config.pisces "/opt/miner/releases/$HELIUM_GA_RELEASE/sys.config"
	OVERRIDE_CONFIG_URL="${ROCKPI_MINER_CONFIG_URL:-https://helium-assets.nebra.com/docker.config.pisces}"
	fi

We should move to just variant as it's more reliable...

Ref #75 (comment)