nixos/rustus: add user name to enable restoring of backups

nixos/modules/services/web-servers/rustus.nix

···
       serviceConfig = {
         ExecStart = "${pkgs.rustus}/bin/rustus";
         StateDirectory = "rustus";
+        # User name is defined here to enable restoring a backup for example
+        # You will run the backup restore command as sudo -u rustus in order
+        # to have write permissions to /var/lib
+        User = "rustus";
         DynamicUser = true;
         LoadCredential = lib.optionals isHybridS3 [
           "S3_ACCESS_KEY_PATH:${cfg.storage.s3_access_key_file}"