buster: apt update config prompt blocks application installations

First report: https://discuss.freedombox.org/t/error-installing-apps/249

Installing apps fails on FreedomBox devices with release set to testing or buster (before Debian 10 release).

Error when installing an app on testing (on demo server)
Error installing application: Error during installation E: The repository 'http://security.debian.org testing/updates Release' no longer has a Release file. E: Repository 'http://cloudfront.debian.net/debian testing InRelease' changed its 'Codename' value from 'buster' to 'bullseye' E: Repository 'http://cloudfront.debian.net/debian testing-updates InRelease' changed its 'Codename' value from 'buster-updates' to 'bullseye-updates'

Error when installing an app on buster (old installation)
Error installing application: Error during installation E: Repository ‘http://security.debian.org/debian-security buster/updates InRelease’ changed its ‘Suite’ value from ‘testing’ to ‘stable’

In both cases, the user has to answer a prompt provided during "apt update".