ci: fix the payload-size checking scripts (#20683)

The scripts were accidentally broken in #20524. More specifically, when a limit
was exceeded the script would break while trying to log an error message due to
a missing `commit` variable.
This commit fixes it and also does some minor clean-up (improve docs, use more
descriptive variable names, remove dead code, etc).

PR Close #20683

aio/scripts/payload.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 set -eu -o pipefail