Merge pull request #46370 from primeos/nixos-sks

sks,nixos/sks: Various minor improvements

Module:
- Add a webroot option (serve a website by default, makes the setup easier)
- Explicitly set a group (instead of using 65534/nogroup)
- Move pkgs.sks from environment.systemPackages to the "sks" user and add pkgs.db

Package:
- Adapt the output of "sks version" to NixOS
- Copy the example webpages to $webSamples