Properly disable apt timers (#13959)

debian/debian/postinst

@@ -73,6 +73,8 @@ systemctl mask libvirtd.socket libvirtd-ro.socket libvirtd-admin.socket libvirtd
 systemctl mask exim4-base.service exim4.service exim4-base.timer
 systemctl mask uuidd.service uuidd.socket
 systemctl mask ndctl-monitor.service
+# Prevent `apt.systemd.daily[3756924]: /usr/lib/apt/apt.systemd.daily: 325: apt-config: Permission denied`
+systemctl mask apt-daily.timer apt-daily-upgrade.timer
 
 # mdmonitor.service in particular causes mdadm to send emails to the MAILADDR line
 # in /etc/mdadm/mdadm.conf. By default, that's the root account, so end-users are