Add support for recurseForDerivations

This will respect `recurseForDerivations` when iterating over attrsets.

Example expression:
``` nix
{ system ? builtins.currentSystem }:
{
  recurseForDerivations = true;

  # This should build as it's in the top-level attrset
  drvA = derivation {
    inherit system;
    name = "drvA";
    builder = ":";
  };

  dontRecurse = {
    # This shouldn't build as `recurseForDerivations = true;` is not set
    # recurseForDerivations = true;

    # This should not build
    drvB = derivation {
      inherit system;
      name = "drvA";
      builder = ":";
    };
  };

  recurse = {
    # This should build
    recurseForDerivations = true;

    # This should not build
    drvC = derivation {
      inherit system;
      name = "drvC";
      builder = ":";
    };
  };

}
```

tests/test_eval.py

@@ -23,7 +23,7 @@ def common_test(extra_args: List[str]) -> None:
         )
 
         results = [json.loads(r) for r in res.stdout.split("\n") if r]
-        assert len(results) == 3
+        assert len(results) == 4
 
         built_job = results[0]
         assert built_job["attr"] == "builtJob"
@@ -32,11 +32,14 @@ def common_test(extra_args: List[str]) -> None:
         assert built_job["drvPath"].endswith(".drv")
         assert built_job["meta"]['broken'] is False
 
-        nested_job = results[1]
-        assert nested_job["attr"] == "nested.job"
-        assert nested_job["name"].startswith("hello-")
+        recurse_drv = results[1]
+        assert recurse_drv["attr"] == "recurse.drvB"
+        assert recurse_drv["name"] == "drvB"
 
-        substituted_job = results[2]
+        recurse_recurse_bool = results[2]
+        assert "error" in recurse_recurse_bool
+
+        substituted_job = results[3]
         assert substituted_job["attr"] == "substitutedJob"
         assert substituted_job["name"].startswith("hello-")
         assert substituted_job["meta"]['broken'] is False