feat(tvix/eval): context-aware dirOf

Change-Id: If73a82a7106de9b479c950741efb70bffabd470a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10424
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/eval/src/builtins/mod.rs

@@ -311,7 +311,9 @@ mod pure_builtins {
         if is_path {
             Ok(Value::Path(Box::new(result.into())))
         } else {
-            Ok(result.into())
+            Ok(Value::String(NixString::new_inherit_context_from(
+                &str, result,
+            )))
         }
     }