mirrordirector will direct ipv6-only host to ipv4 mirror

[axoin]

When running pi-gen, the bootstrap will fail, because some packages are unretrievable.

It would be better the mirrordirector directs to ipv6 only when asked on ipv6, or at least use dual-stack mirrors.

```
root@vServer:~/pi-gen# wget http://mirrordirector.raspbian.org/raspbian/pool/main/i/init-system-helpers/init_1.60_armhf.deb
--2022-10-13 12:42:54-- http://mirrordirector.raspbian.org/raspbian/pool/main/i/init-system-helpers/init_1.60_armhf.deb
Resolving mirrordirector.raspbian.org (mirrordirector.raspbian.org)... 2a00:1098:0:80:1000:75:0:3, 93.93.128.193
Connecting to mirrordirector.raspbian.org (mirrordirector.raspbian.org)|2a00:1098:0:80:1000:75:0:3|:80... connected.
HTTP request sent, awaiting response... 302 Found
Location: http://ftp.agdsn.de/pub/mirrors/raspbian/raspbian/pool/main/i/init-system-helpers/init_1.60_armhf.deb [following]
--2022-10-13 12:42:55-- http://ftp.agdsn.de/pub/mirrors/raspbian/raspbian/pool/main/i/init-system-helpers/init_1.60_armhf.deb
Resolving ftp.agdsn.de (ftp.agdsn.de)... 141.30.235.39
Connecting to ftp.agdsn.de (ftp.agdsn.de)|141.30.235.39|:80... failed: Network is unreachable.
```

```
$ host ftp.agdsn.de
ftp.agdsn.de has address 141.30.235.39
```

Shouldn't your hosts be at least all dual-stack ?

EDIT: Some more failing (out of apparent 28?)
```
mirror.us.leaseweb.net (mirror.us.leaseweb.net)|207.244.94.80 |:80... failed: Network is unreachable.
quantum-mirror.hu (quantum-mirror.hu) |84.2.241.164 |:80... failed: Network is unreachable.
raspbian.mirror.net.in (raspbian.mirror.net.in)|103.109.86.3 |:80... failed: Network is unreachable.
raspbian.mirror.net.in (raspbian.mirror.net.in)|14.192.2.114 |:80... failed: Network is unreachable.
raspbian.mirror.net.in (raspbian.mirror.net.in)|45.126.190.120 |:80... failed: Network is unreachable.
raspbian.mirror.vu.lt (raspbian.mirror.vu.lt) |158.129.159.169|:80... failed: Network is unreachable.
```