Preserve UIDs and GIDs between reboots on ephemeral systems (#101)

On ephemeral systems, users and groups are recreated on each boot.
Therefore, adding new users or groups may shuffle UIDs and GIDs and
break persisted file permissions.

Persisting /var/lib/nixos is sufficient to preserve UIDs and GIDs
between boot.

hosts/marlon/default.nix

@@ -42,6 +42,7 @@
 
     directories = [
       "/var/log"
+      "/var/lib/nixos"  # preserve uids/gids between reboots
     ];
   };
 }

hosts/roland/default.nix

@@ -40,6 +40,7 @@
 
     directories = [
       "/var/log"
+      "/var/lib/nixos"  # preserve uids/gids between reboots
     ];
   };
 }