* Grants enough privileges to the configured user so that it can run
mysqldump.
* Adds a nixos test.
* Use systemd timers instead of a cronjob (by @fadenb).
* Creates a new user for backups by default, instead of using mysql
user.
* Ensures that backup user has write permissions on backup location.
* Write backup to a temporary file before renaming so that a failed
backup won't overwrite the previous backup, and so that the backup
location will never contain a partial backup.
Breaking changes:
* Renamed period to calendar to reflect the change in how to
configure the backup time.
* A failed backup will no longer result in cron sending an e-mail --
users' monitoring systems must be updated.
Resolves #24728
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
