phonebox: stop dnsmasq trying to write leases to /dev/null

dhcp-leasefile=/dev/null logs 'failed to write /dev/null: Invalid argument'
every lease; leasefile-ro is the supported way to run without a lease file.

modules/clan/phonebox/default.nix

@@ -287,7 +287,7 @@
                 dhcp-range = [
                   "192.168.254.100,192.168.254.100,255.255.255.0,3m"
                 ];
-                dhcp-leasefile = "/dev/null";
+                leasefile-ro = true;
                 dhcp-option = [
                   "3,192.168.254.1"
                 ];