Sets unattended-upgrades reboot time to "now"

"Now" was the default value, we previously set it to the
'daily_reboot_time' to provide some predictability around reboots, but
that came at the cost of separating the package updates from the reboot
logic. Ideally, we'll have as narrow gap as possible between:

  * apt update
  * apt upgrade
  * reboot

and these changes implement that.

install_files/ansible-base/roles/common/templates/80securedrop.j2

@@ -1,8 +1,7 @@
 {% if ansible_distribution_release == "focal" %}
-// If automatic reboot is enabled and needed, reboot at the specific
-// time instead of immediately
-//  Default: "now"
-Unattended-Upgrade::Automatic-Reboot-Time "{{ daily_reboot_time }}:00";
+// Reboot should happen after nightly upgrades. Timing of upgrade
+// is configured via apt.daily.timer
+Unattended-Upgrade::Automatic-Reboot-Time "now";
 {% endif %}
 // Don't install packages from "Recommends" field, we'll manage dependencies
 // explicitly to avoid pulling in packages from e.g. universe.