fix(tvix/eval): allow use of ? operator on non-set types

Nix allows this, but always returns false. Tvix needs to do the same.

Change-Id: Ic9eec90834a0d0969eea5316d5c25032d3691d94
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6209
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

tvix/eval/src/value/mod.rs

@@ -116,7 +116,7 @@ impl Display for Value {
             }
 
             // internal types
-            Value::AttrPath(_) => f.write_str("internal[attrpath]"),
+            Value::AttrPath(path) => write!(f, "internal[attrpath({})]", path.len()),
             Value::Blackhole => f.write_str("internal[blackhole]"),
             Value::NotFound => f.write_str("internal[not found]"),
         }