fix(tvix/eval): emit correct count in OpAttrPath

Not sure how exactly this snuck in, but it caused some subtle
breakages in deeply nested attribute sets.

Change-Id: I8049ce912405d3750031f79cc8d86ff1c3c02c2b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6208
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

tvix/eval/src/value/attrs.rs

@@ -253,7 +253,7 @@ impl NixAttrs {
                     continue;
                 }
 
-                other => panic!("unexpected attribute key type: {}", other.type_of()),
+                other => panic!("unexpected attribute key: {} :: {}", other, other.type_of()),
             }
         }