Shuffle validator order in download_missing_sender_blocks (#4724)

## Motivation

Right now we're going through the validators always in the same order,
which is not ideal

## Proposal

Shuffle the validator order

## Test Plan

CI

## Release Plan

- These changes should be backported to the latest `testnet` branch,
then
    - be released in a validator hotfix.

Author: ndr-ds

linera-core/src/client/mod.rs

@@ -57,7 +57,10 @@ use linera_execution::{
 };
 use linera_storage::{Clock as _, ResultReadCertificates, Storage as _};
 use linera_views::ViewError;
-use rand::distributions::{Distribution, WeightedIndex};
+use rand::{
+    distributions::{Distribution, WeightedIndex},
+    seq::SliceRandom,
+};
 use serde::{Deserialize, Serialize};
 use thiserror::Error;
 use tokio::sync::{mpsc, OwnedRwLockReadGuard};
@@ -2323,14 +2326,15 @@ impl<Env: Environment> ChainClient<Env> {
         stream::iter(missing_blocks.into_iter())
             .map(|(sender_chain_id, heights)| {
                 let height = heights.into_iter().max();
-                let nodes = nodes.clone();
                 let this = self.clone();
+                let mut shuffled_nodes = nodes.clone();
+                shuffled_nodes.shuffle(&mut rand::thread_rng());
                 async move {
                     let Some(height) = height else {
                         return Ok(());
                     };
                     // Try to download from any node.
-                    for node in &nodes {
+                    for node in &shuffled_nodes {
                         if let Err(err) = this
                             .download_sender_block_with_sending_ancestors(
                                 sender_chain_id,