client-server autopkgtest fails: Failed to set LLMNR configuration: Link 3 not known

[Benjamin Drung]

client-server autopkgtest fails. Relevant log:

```
115s + ip netns exec client dhclient ptp-veth-client
116s Setting LLMNR support level "yes" for "3", but the global support level is "no".
116s Failed to set LLMNR configuration: Link 3 not known
116s Failed to set MulticastDNS configuration: Link 3 not known
116s Failed to set domain configuration: Link 3 not known
116s Failed to set DNS configuration: Link 3 not known
116s + ip netns exec client ip addr
116s 1: lo: <LOOPBACK> mtu 65536 qdisc noop state DOWN group default qlen 1000
116s link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
116s 3: ptp-veth-client@if4: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
116s link/ether 26:d9:c1:82:7f:fa brd ff:ff:ff:ff:ff:ff link-netns server
116s inet 10.42.42.10/24 brd 10.42.42.255 scope global dynamic ptp-veth-client
116s valid_lft 600sec preferred_lft 600sec
116s inet6 fe80::24d9:c1ff:fe82:7ffa/64 scope link
116s valid_lft forever preferred_lft forever
116s + grep 1.1.1.1 /etc/netns/client/resolv.conf
```

Full log: https://autopkgtest.ubuntu.com/results/autopkgtest-noble/noble/amd64/i/isc-dhcp/20231127_150240_2bc90@/log.gz