fix bug with for generator variables in mixin (#297)

pkl-core/src/test/files/LanguageSnippetTests/input/generators/forGeneratorInMixins.pkl

@@ -38,3 +38,13 @@ function mapEnvLiteral(_env: Dynamic) = (it) -> (it) {
     }
   }
 }
+
+function addElements(keys: List<String>): Mixin<Mapping<String, String>> = new {
+  for (key in keys) {
+    [key] = key
+  }
+}
+
+res = new Mapping<String, String> {
+  ["base"] = "alreadyThere"
+} |> addElements(List("newElement"))

pkl-core/src/test/files/LanguageSnippetTests/output/generators/forGeneratorInMixins.pcf

@@ -36,3 +36,7 @@ foo {
     }
   }
 }
+res {
+  ["base"] = "alreadyThere"
+  ["newElement"] = "newElement"
+}