Motivation for this change

The default for logFile is /var/log/couchdb.log, and the tmpfile rules chown
${dirOf cfg.logFile}, which is just /var/log, to couchdb:couchdb.

This was found by Edes' report on IRC, which looked like

Detected unsafe path transition /var/log → /var/log/journal during canonicalization of /var/log/journal

While this bug has been present since the initial couchdb module in
62438c0 by @garbas, this wasn't a
problem, because the initial module only created and chowned /var/log
if it didn't exist yet, which can't occur because this gets created in
the initial phases of NixOS startup.

However with the recent move from manual preStart chown scripts to
systemd.tmpfiles.rules in 062efe0 (#59389),
this chown is suddenly running unconditionally at every system
activation, therefore triggering the above error.

So I'm glad @aanderse pushed for this, otherwise we wouldn't have found this bug :)

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.