feat(nix/eval): Implement builtins.functionArgs

Now that we're tracking formals on Lambda this ends up being quite easy;
we just pull them off of the Lambda for the argument closure and use
them to construct the result attribute set.

Change-Id: I811cb61ec34c6bef123a4043000b18c0e4ea0125
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7003
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/value/attrs.rs

@@ -127,6 +127,10 @@ mod arbitrary {
 }
 
 impl NixAttrs {
+    pub fn empty() -> Self {
+        Self(AttrsRep::Empty)
+    }
+
     /// Return an attribute set containing the merge of the two
     /// provided sets. Keys from the `other` set have precedence.
     pub fn update(self, other: Self) -> Self {