Fix time sync: wait for chrony to actually sync before proceeding

chronyc makestep only applies a correction if chrony already has a
time estimate, but on first boot chrony may not have completed NTS-KE
handshake yet. Replace with waitsync which blocks until chrony has
synced with at least one source (up to 30s timeout, 1s threshold).

Author: kvinwang

basefiles/dstack-prepare.sh

@@ -95,9 +95,8 @@ mount_overlay /bin $OVERLAY_TMP
 mount_overlay /home $OVERLAY_TMP
 
 # Make sure the system time is synchronized
-log "Syncing system time..."
-# Let the chronyd correct the system time immediately
-chronyc makestep
+log "Waiting for time sync..."
+chronyc waitsync 30 1 0 0 2>/dev/null || log "Warning: time sync timed out"
 
 if ! [[ -e /dev/tdx_guest ]]; then
 	modprobe tdx-guest