Improve time sync logging visibility

Add retry loop with per-attempt logging for NTS time sync, making it
easy to diagnose slow NTS-KE handshakes in boot logs.

Author: kvinwang

dstack-util/src/system_setup.rs

@@ -708,10 +708,18 @@ pub async fn cmd_sys_setup(args: SetupArgs) -> Result<()> {
 async fn do_sys_setup(stage0: Stage0<'_>) -> Result<()> {
     if stage0.shared.app_compose.secure_time {
         info!("Waiting for the system time to be synchronized");
-        cmd! {
-            chronyc waitsync 20 0.1;
+        let mut synced = false;
+        for i in 1..=30 {
+            if cmd! { chronyc waitsync 1 0.2; }.is_ok() {
+                synced = true;
+                break;
+            }
+            info!("Time sync attempt {i}/30 failed, retrying...");
+            tokio::time::sleep(Duration::from_secs(1)).await;
+        }
+        if !synced {
+            bail!("Failed to sync system time after 30 attempts");
         }
-        .context("Failed to sync system time")?;
     } else {
         info!("System time will be synchronized by chronyd in background");
     }