onepublish: accept a new environnment variable for `config[:external_url]`

I my test job #28385, I have image source http://192.168.230.55:36063/debian/10.10.0/base/debian-10.10.0.img.

Looking in the code:

• BUILD_ADDR should be automatically determined from runner environment (IP of first non lo interface) instead of requiring a configuration from the config file or environment variable
• config[:external_url] should have an EXTERNAL_URL environment variable