Merge pull request #15166 from NixOS/backport-15160-to-2.33-maintenance

[Backport 2.33-maintenance] builtins.flakeRefToString: Evaluate attributes

src/libflake/flake-primops.cc

@@ -128,6 +128,7 @@ static void prim_flakeRefToString(EvalState & state, const PosIdx pos, Value **
     state.forceAttrs(*args[0], noPos, "while evaluating the argument passed to builtins.flakeRefToString");
     fetchers::Attrs attrs;
     for (const auto & attr : *args[0]->attrs()) {
+        state.forceValue(*attr.value, attr.pos);
         auto t = attr.value->type();
         if (t == nInt) {
             auto intValue = attr.value->integer().value;