Fix parallel command

flake.nix

@@ -23,7 +23,7 @@
           '';
           myUsersTest = pkgs.writeShellScriptBin "users-test" ''
             pushd ${myUsersDir} > /dev/null &&
-            ${pkgs.parallel}/bin/parallel :: ${myUsersDir}/bin/users-server ${myUsersDir}/bin/users-gateway
+            ${pkgs.parallel}/bin/parallel ::: ${myUsersDir}/bin/users-server ${myUsersDir}/bin/users-gateway
             popd > /dev/null
           '';
       in