fix(tvix/eval): context-aware dirOf

`dirOf` forgot to accepts contextful strings, e.g. derivations and propagates this context
further.

Change-Id: I6c05944a3ce5073e243e7676c9be56c48407d657
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10618
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/eval/src/builtins/mod.rs

@@ -375,7 +375,7 @@ mod pure_builtins {
                 span,
             )
             .await?
-            .to_str()?;
+            .to_contextful_str()?;
         let result = str
             .rsplit_once('/')
             .map(|(x, _)| match x {