refactor(tvix/eval): get rid of Value::Blackhole variant

This is no longer needed for anything and the extra clone here is not
really more costly than constructing a blackhole value in a different
place.

Change-Id: I5c63085b1b4418b629ea58a42e3bfe9a9b586d76
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6275
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

tvix/eval/src/value/attrs.rs

@@ -315,8 +315,8 @@ fn attempt_optimise_kv(slice: &mut [Value]) -> Option<NixAttrs> {
     };
 
     Some(NixAttrs(AttrsRep::KV {
-        name: std::mem::replace(&mut slice[name_idx], Value::Blackhole),
-        value: std::mem::replace(&mut slice[value_idx], Value::Blackhole),
+        name: slice[name_idx].clone(),
+        value: slice[value_idx].clone(),
     }))
 }