firewall: ensure wireguard egress traffic uses the anycast source IP

Before we relied on the IP being first in the interfaces file, which
is less than optimal. Now we use nftables to ensure the correct source
IP is set only for the (fwmarked) wireguard traffic.

Also remove iface hints from interfaces configuration as they are not
needed with ifupdown-ng.

roles/firewall/templates/interfaces.j2

@@ -1,10 +1,9 @@
 {% set addrs = interfaces | selectattr('name', '==', 'lo') | map(attribute='ip_addresses') | first -%}
 
-source-directory /etc/network/interfaces.d
-
 auto lo
-iface lo inet loopback
-    address {{ wg_ip }}
+iface lo
 {% for address in addrs %}
     address {{ address.address }}
 {% endfor %}
+
+source-directory /etc/network/interfaces.d