bug#52533: guix deploy breaks SSH access with a PAM error

[Maxim Cournoyer <maxim.cournoyer@gmail.com>]

Hello Guix!

Following the big merge of the core-updates-frozen branch into master,
I've noticed now on two counts the following: running 'guix deploy'
leaves the remote machine unreachable by SSH.  The connection passes
authentication but then gets closed immediately.  /var/log/messages
reveals the following error:

--8<---------------cut here---------------start------------->8---
sshd[29578]:  error: PAM: pam_open_session(): Module is unknown
--8<---------------cut here---------------end--------------->8---

The machines updated were running Guix System revisions predating the
core-updates-frozen merge.

The 'guix deploy' command doesn't succeed due to SSH starting to fail at
99% completion or similar; the bootloader configuration is not updated
so rebooting boots into the same old system generation (and SSH works
again):

--8<---------------cut here---------------start------------->8---
guix deploy: deploying to x200...
guix deploy: sending 0 store items (0 MiB) to 'x200.local'...
guix deploy: sending 0 store items (0 MiB) to 'x200.local'...
substitute: updating substitutes from 'http://127.0.0.1:8181'... 100.0%
substitute: updating substitutes from 'https://ci.guix.gnu.org'... 100.0%
The following derivations will be built:
   /gnu/store/049wr939gjpgl3471wrk8b1waqgswrdi-remote-exp.scm.drv
   /gnu/store/y1mgddpa2qkrmc01knpdam917b60yxlq-switch-to-system.scm.drv
   /gnu/store/vgadszcfklbhr7d8yl8jprzipjy6b0vj-system.drv
   /gnu/store/ypyaf6ib1w5nc4kr0xgjm4par407cnzk-provenance.drv

building /gnu/store/ypyaf6ib1w5nc4kr0xgjm4par407cnzk-provenance.drv...
building /gnu/store/vgadszcfklbhr7d8yl8jprzipjy6b0vj-system.drv...
building /gnu/store/y1mgddpa2qkrmc01knpdam917b60yxlq-switch-to-system.scm.drv...
building /gnu/store/049wr939gjpgl3471wrk8b1waqgswrdi-remote-exp.scm.drv...
guix deploy: sending 5 store items (0 MiB) to 'x200.local'...
guix deploy: error: failed to deploy x200: failed to start 'guix repl' on 'x200.local'

$ guix deploy ~/stow/guix/machines/x200.scm --no-offload
The following 1 machine will be deployed:
  x200

guix deploy: deploying to x200...
guix deploy: error: failed to deploy x200: remote command
'/run/setuid-programs/sudo -n -- guix repl -t machine' failed with
status 254

$ ssh x200
Last login: Wed Dec 15 23:28:02 2021 from 192.168.10.15
Connection to x200.local closed.
--8<---------------cut here---------------end--------------->8---

This is obviously embarrassing in scenarios where the SSH connection is
the main way to reach to the remote machine.

Ideas?

Thank you,

Maxim