Bug#954321: in the Debian AWS-images cloud-init network fails if ipv6 is disabled

[Bas Zoetekouw <bas@debian.org>]

    package: cloud.debian.org
    
    (I previously reported this bug again cloud-init, see https://bugs.launchpad.net/cloud-init/+bug/1863773,
    but it seems it is instead an issue with the Debian images
    themselves).
    
    I'm using cloud-init on a Debian machien in AWS using the official
    Debian buster images (https://wiki.debian.org/Cloud/AmazonEC2Image/Buster)

On these machines, ipv6 is disabled by setting /proc/sys/net/ipv6/conf/all/disable_ipv6 to 1. This causes the network to fail:

bas@machine:/etc/network$ sudo ifup ens5
Internet Systems Consortium DHCP Client 4.4.1
Copyright 2004-2018 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/

Listening on LPF/ens5/02:45:ca:0d:93:76
Sending on LPF/ens5/02:45:ca:0d:93:76
Sending on Socket/fallback
DHCPREQUEST for 10.0.16.20 on ens5 to 255.255.255.255 port 67
DHCPACK of 10.0.16.20 from 10.0.16.1
RTNETLINK answers: File exists
bound to 10.0.16.20 -- renewal in 1796 seconds.

Could not get a link-local address
run-parts: /etc/network/if-pre-up.d/cloud_inet6 exited with return code 1
ifup: failed to bring up ens5

Even worse, on boot the machines do get a proper ipv4 address, but (apparently because setting up the network fails), the dhcp (v4) daemon is stopped, and the machines "mysteriously" drop off the net after the lease ends.