refactor(tvix/eval): address clippy lints

Change-Id: Ic2bd4e8291b30ceac9fa0e88a4f56e61ae99b603
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10227
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/eval/builtin-macros/tests/tests.rs

@@ -15,7 +15,7 @@ mod builtins {
     }
 
     #[builtin("tryEval")]
-    pub async fn builtin_try_eval(co: GenCo, #[lazy] _x: Value) -> Result<Value, ErrorKind> {
+    pub async fn builtin_try_eval(_co: GenCo, #[lazy] _x: Value) -> Result<Value, ErrorKind> {
         unimplemented!("builtin is never called")
     }
 }