Fix waitsync loop to properly wait for NTS handshake completion

The previous single waitsync call with 30 retries completed in ~3s,
too fast for NTS-KE TLS handshake. Use a loop with 1s sleep between
attempts, allowing up to 30s total for sync.

Author: kvinwang

basefiles/dstack-prepare.sh

@@ -96,7 +96,13 @@ mount_overlay /home $OVERLAY_TMP
 
 # Make sure the system time is synchronized
 log "Waiting for time sync..."
-chronyc waitsync 30 0.2 0 0 2>/dev/null || log "Warning: time sync timed out"
+for i in $(seq 1 30); do
+	if chronyc waitsync 1 0.2 0 0 2>/dev/null; then
+		log "Time synchronized"
+		break
+	fi
+	sleep 1
+done || log "Warning: time sync timed out"
 
 if ! [[ -e /dev/tdx_guest ]]; then
 	modprobe tdx-guest