backups: backup NixOS uid, gid mappings

Now that service uid, gid mappings are included in the backups, along
with the service data dirs, we can remove 'chown -R' for
clightning and liquidd data dirs.

Note that we used 'chown -R' only for these two services, while this
approach would have been relevant for all services with data dirs.

modules/backups.nix

@@ -21,6 +21,8 @@ let
     ${config.services.postgresqlBackup.location}/btcpaydb.sql.gz
     ${optionalString config.nix-bitcoin.generateSecrets "${config.nix-bitcoin.secretsDir}"}
     /var/lib/tor
+    /var/lib/nixos
+
     # Extra files
     ${cfg.extraFiles}