fix(tvix/eval): Fail on duplicate attribute set keys

Change-Id: I57373ca76d0e25a5d08a8dfce9d5949099326fc0
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6104
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/errors.rs

@@ -2,6 +2,10 @@ use std::fmt::Display;
 
 #[derive(Debug)]
 pub enum Error {
+    DuplicateAttrsKey {
+        key: String,
+    },
+
     TypeError {
         expected: &'static str,
         actual: &'static str,