fix regression: setting 3rd party URL via ARDUINO_BOARD_MANAGER_ADDITIONAL_URLS env var

[cmaglie]

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What kind of change does this PR introduce?

Fixes a regression of release 1.0.0: setting more than one URL via ARDUINO_BOARD_MANAGER_ADDITIONAL_URLS was impossible.

What is the current behavior?

$ export ARDUINO_BOARD_MANAGER_ADDITIONAL_URLS="https://espressif.github.io/arduino-esp32/package_esp32_index.json https://arduino.esp8266.com/stable/package_esp8266com_index.json"
$ arduino-cli config get board_manager.additional_urls
- https://espressif.github.io/arduino-esp32/package_esp32_index.json https://arduino.esp8266.com/stable/package_esp8266com_index.json

The configuration is incorrect because the two URLs are concatenated in a single string.

What is the new behavior?

$ export ARDUINO_BOARD_MANAGER_ADDITIONAL_URLS="https://espressif.github.io/arduino-esp32/package_esp32_index.json https://arduino.esp8266.com/stable/package_esp8266com_index.json"
$ arduino-cli config get board_manager.additional_urls
- https://espressif.github.io/arduino-esp32/package_esp32_index.json
- https://arduino.esp8266.com/stable/package_esp8266com_index.json

Does this PR introduce a breaking change, and is titled accordingly?

Other information

Fix #2643