feat(umber): automatically shutdown

The power to umber turns off at midnight. Therefore, we should shut it
down beforehand to make sure it shuts down cleanly

packetmix/systems/umber/auto-shutdown.nix

···
+{ pkgs, ... }:
+{
+  systemd.timers."auto-shutdown" = {
+    wantedBy = [ "timers.target" ];
+    timerConfig = {
+      OnCalendar = "* *-*-* 23:55:00 Etc/UTC";
+      Persistent = false;
+      Unit = "auto-shutdown.service";
+    };
+  };
+
+  systemd.services."auto-shutdown" = {
+    script = ''
+      ${pkgs.systemd}/bin/systemctl poweroff
+    '';
+    serviceConfig = {
+      RemainAfterExit = true;
+      Type = "oneshot";
+      User = "root";
+    };
+  };
+}