Default user for backup has been changed from `backup` to `borg`.

User `backup` is now a system user on Debian testing. After each upgrade
involving related pacakges (pam?) our `backup` user is overwritten by
the Debian system one.

Also the default repositories folder has been changed from
`/home/backup/repos` to `/home/borg/repos`.

To adapt our existing infrastructure, after moving all the repos for all
the servers, some metadata (cache probably) need to be updated.

This update is done automatically when creating a new archvie if we set
the environment variable `BORG_RELOCATED_REPO_ACCESS_IS_OK` to `yes`.

Our backup script has been adapted to set this env variable to `yes`,
then we run a first run of backup on each host, afterwards we changed
the env variable in all backup scripts again to `no`.