postinst fails due to nss upgrade in proposed?

This could to be very much the same as bug 1746947 (might dup once sure)

Set up Bionic like autopkgtest does:
Use Pinning to get the autopkgtest style:
# cat /etc/apt/preferences.d/nssonlyproposed
Package: *
Pin: release a=bionic
Pin-Priority: 1001
Package: libnss3 libnss3-tools libnss3-dev libnss3-dbg
Pin: release a=bionic-proposed
Pin-Priority: 1002

Then on installing corosync-qnetd I get this:

Setting up corosync-qnetd (2.4.2-3build1) ...
Creating /etc/corosync/qnetd/nssdb
Creating new key and cert db
password file contains no data
stat: cannot read file system information for '%u:%g': No such file or directory
chmod: cannot access '/etc/corosync/qnetd/nssdb/key3.db': No such file or directory
chmod: cannot access '/etc/corosync/qnetd/nssdb/cert8.db': No such file or directory
chmod: cannot access '/etc/corosync/qnetd/nssdb/secmod.db': No such file or directory
Creating new noise file /etc/corosync/qnetd/nssdb/noise.txt
Creating new CA

Generating key. This may take a few moments...

Is this a CA certificate [y/N]?
Enter the path length constraint, enter to skip [<0 for unlimited path]: > Is this a critical extension [y/N]?

Generating key. This may take a few moments...

Notice: Trust flag u is set automatically if the private key is present.
QNetd CA certificate is exported as /etc/corosync/qnetd/nssdb/qnetd-cacert.crt
chgrp: cannot access '/etc/corosync/qnetd/nssdb/cert8.db': No such file or directory
chgrp: cannot access '/etc/corosync/qnetd/nssdb/key3.db': No such file or directory