[cron] sleep random seconds (<59), if not interactive or forced

https://github.com/acmesh-official/acme.sh/pull/944#issuecomment-707255200

Let's Encrypt employee said in the comments "we do see peaks at the beginning of minutes and even seconds; the finer-grained time randomization, the better."

This adds a random amount of sleep second before beginning the cron job. I considered reading from `/dev/urandom` and so on, but we aren't doing anything security critical here so I thought that just using the process number modulo 59 (the largest prime <= 60) should give decent variability across the systems. The starting hour and minute are already randomized during the installation.

acme.sh

@@ -6737,6 +6737,11 @@ cron() {
   export _ACME_IN_CRON=1
   _initpath
   _info "$(__green "===Starting cron===")"
+  if [ -z "$FORCE" ] && [ -z "$__INTERACTIVE" ]; then
+    random_sec=$(_math $$ % 59)
+    _info "Sleeping for $random_sec seconds."
+    _sleep $random_sec
+  fi
   if [ "$AUTO_UPGRADE" = "1" ]; then
     export LE_WORKING_DIR
     (