update-device-vars: Add a sleep to avoid 429: Too Many Requests

Change-Id: Ib4a382cb4b1af025b401e54b07d69414c419f4cf

pixel/update-device-vars.sh

@@ -81,6 +81,7 @@ main() {
       sed -i "/ ota_sha256=/c\readonly ota_sha256=\"$new_ota_sha256\"" "${dv}"
       sed -i "/ security_patch=/c\readonly security_patch=\"$new_security_patch\"" "${dv}"
     )
+    sleep 10s
   done
 }