connect: fix ipv6 bind_address test

[gulducat]

I realized this test I wrote in #24203 didn't actually test the IPv6-detection logic, because "bind_address": "::" was being passed in as config, effectively bypassing the important bit here:

nomad/command/agent/consul/connect.go

Lines 250 to 252 in eb8d240

	if _, ok := cfg["bind_address"]; !ok {
	cfg["bind_address"] = connectProxyBindAddress(networks)
	}