Stop daily spam from ecours to sysadmins due to disabled IPv6

Originally created by @intrigeri on #11342 (Redmine)

Apparently, the big hammer approach that was taken to disable IPv6 on ecours (commit f402038 in the puppet-tails module) has unfortunate side-effects:

/etc/cron.daily/reboot-notifier:
send-mail: warning: inet_protocols: disabling IPv6 name/address support: Address family not supported by protocol
postdrop: warning: inet_protocols: disabling IPv6 name/address support: Address family not supported by protocol

Can you please address that one?

Parent Task: #8647 (closed)