refactor(tvix/eval): add NixAttrs::contains function

This avoids copying around the value more than needed.

Change-Id: I35949d16dad7fb8f76e0f641eaccf48322144777
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6263
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/vm.rs

@@ -242,7 +242,7 @@ impl VM {
                 OpCode::OpAttrsIsSet => {
                     let key = self.pop().to_string()?;
                     let result = match self.pop() {
-                        Value::Attrs(attrs) => attrs.select(key.as_str()).is_some(),
+                        Value::Attrs(attrs) => attrs.contains(key.as_str()),
 
                         // Nix allows use of `?` on non-set types, but
                         // always returns false in those cases.