feat(tvix/eval): Implement builtins.seq

Since we already have infra for forcing arguments to builtins, this ends
up being almost *too* simple - we just return the second argument!

Change-Id: I070d3d0b551c4dcdac095f67b31e22e0de90cbd7
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6999
Reviewed-by: kanepyork <rikingcoding@gmail.com>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/builtins/mod.rs

@@ -490,6 +490,11 @@ fn pure_builtins() -> Vec<Builtin> {
                 Ok(Value::attrs(NixAttrs::from_map(res)))
             },
         ),
+        Builtin::new("seq", &[true, true], |mut args: Vec<Value>, _: &mut VM| {
+            // The builtin calling infra has already forced both args for us, so we just return the
+            // second and ignore the first
+            Ok(args.pop().unwrap())
+        }),
         Builtin::new("splitVersion", &[true], |args: Vec<Value>, _: &mut VM| {
             let s = args[0].to_str()?;
             let s = VersionPartsIter::new(s.as_str());