NixOS audit system must protect login UIDs from unauthorized change.

If audit information were to become compromised, then forensic analysis and discovery of the true source of potentially malicious system activity is impossible to achieve. Audit information includes all information (e.g., audit records, audit settings, audit reports) needed to successfully audit system activity. In immutable mode, unauthorized users cannot execute changes to the audit system to potentially hide malicious activity and then put the audit rules back. A system reboot would be noticeable and a system administrator could then investigate the unauthorized changes. Satisfies: SRG-OS-000058-GPOS-00028, SRG-OS-000059-GPOS-00029