bug#36389: nginx/certbot interaction doesn't work as documented

[Robert Vollmert <rob@vllmrt.net>]

I’ve tried setting up nginx with certbot on guix. Two immediate issues:

- certbot extends the nginx service to serve challenge files. It appears
  that this nginx service extension conflicts (silently) with an independently
  configured nginx service. I.e., I had nginx previously configured, and
  after adding certbot, my previous nginx kept running with the previous
  configuration (even after herd restart nginx), while there was an additional
  nginx config in the gnu store with the certbot-specific fragments. certbot
  activation called nginx to test that fragment, but apparently never started
  nginx (successfully?). There were no errors.

  After removing the stand-alone nginx service and restarting nginx, it started
  with the certbot configuration.

- After this, /var/lib/certbot/renew worked successfully to register a
  certificate, but then failed when calling the nginx deploy hook that I’d
  copied from the guix certbot documentation, because /var/run/nginx/pid
  doesn’t exist. That might be a bug in the nginx package, not sure. I can’t
  find an nginx pid file anywhere, and no other errors related to it either,
  even though the config file includes
    pid /var/run/nginx/pid;