Do the builders have working IPv4 localhost?

Yes, they do. From a non-production builder:

```
ubuntu@dogfood-lcy02-amd64-001:~$ host localhost
localhost has address 127.0.0.1
localhost has IPv6 address ::1
ubuntu@dogfood-lcy02-amd64-001:~$ ping -4 -w1 localhost
PING localhost (127.0.0.1) 56(84) bytes of data.
64 bytes from localhost (127.0.0.1): icmp_seq=1 ttl=64 time=0.013 ms

--- localhost ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.013/0.013/0.013/0.000 ms
ubuntu@dogfood-lcy02-amd64-001:~$ ping -6 -w1 localhost
PING localhost(localhost (::1)) 56 data bytes
64 bytes from localhost (::1): icmp_seq=1 ttl=64 time=0.046 ms

--- localhost ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.046/0.046/0.046/0.000 ms
```