feat(tvix/eval): use with_capacity instead of a growing vec

Change-Id: I4d89663eb9ac772ce1008ed5ee218bc7016164e7
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12733
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/eval/src/value/json.rs

@@ -40,7 +40,7 @@ impl Value {
             }
 
             Value::List(l) => {
-                let mut out = vec![];
+                let mut out = Vec::with_capacity(l.len());
 
                 for val in l.into_iter() {
                     let (json_item, ctx) = Box::pin(val.into_contextful_json(co)).await?;