Add configuration for update check to target a different repository

[a-maze-d]

This is a proposal for this idea to solve the issue of nerves_livebook not displaying the correct version or a banner that isn't valid, because more effort is required to get the latest version of livebook into nerves.

[CLAassistant]

All committers have signed the CLA.

[github-actions[bot]]

Uffizzi Preview deployment-55279 was deleted.

[josevalim]

:green_heart: :blue_heart: :purple_heart: :yellow_heart: :heart:

[a-maze-d]

Thanks all