Attempt to fix XLA CI build

Author: lawik

scripts/ci_create_firmware.sh

@@ -17,7 +17,7 @@ set -e
 [[ -n "$MIX_ENV" ]] || MIX_ENV=prod
 
 # For the firmwares that use XLA
-XLA_TARGET_PLATFORM="aarch64-linux-gnu"
+export XLA_TARGET_PLATFORM="aarch64-linux-gnu"
 FULL_FIRMWARE_FILENAME="nerves_livebook_${MIX_TARGET}.fw"
 FULL_FIRMWARE_PATH="$DEPLOY_PATH/$FULL_FIRMWARE_FILENAME"
 PREVIOUS_FIRMWARE_FILENAME="previous_${MIX_TARGET}.fw"