userborn: enable by default

I have tested this for a wile without any issues.
If it works find in srvos, we can propose this to nixos itself.

nixos/common/default.nix

@@ -1,6 +1,6 @@
 # A default configuration that applies to all servers.
 # Common configuration across *all* the machines
-{ config, lib, ... }:
+{ config, lib, options, ... }:
 {
 
   imports = [
@@ -15,6 +15,10 @@
     ./zfs.nix
   ];
 
+  # Create users with https://github.com/nikstur/userborn rather than our perl script.
+  # Don't enable if we detect impermanence, which is not compatible with it: https://github.com/nix-community/impermanence/pull/223
+  services.userborn.enable = lib.mkIf (options.environment ? persistence) (lib.mkDefault true);
+
   # Use systemd during boot as well except:
   # - systems with raids as this currently require manual configuration: https://github.com/NixOS/nixpkgs/issues/210210
   # - for containers we currently rely on the `stage-2` init script that sets up our /etc