fix(tvix/eval): type check function argument with set pattern

C++ Nix forces and typechecks the passed argument even if it is not
necessary in order to compute the return value of the function. I
discovered this when I thought our formals miscompilation might be that
we are too strict, but doesn't look like it in this case.

Change-Id: Ifb3c92592293052c489d1e3ae8c7c54e4b6b4dc6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8701
Tested-by: BuildkiteCI
Autosubmit: sterni <sternenseemann@systemli.org>
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/src/value/mod.rs

@@ -542,6 +542,7 @@ impl Value {
     gen_is!(is_path, Value::Path(_));
     gen_is!(is_number, Value::Integer(_) | Value::Float(_));
     gen_is!(is_bool, Value::Bool(_));
+    gen_is!(is_attrs, Value::Attrs(_));
 
     // TODO(amjoseph): de-asyncify this (when called directly by the VM)
     /// Compare `self` against other using (fallible) Nix ordering semantics.