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-limits.json

@@ -1,3 +1,21 @@
 {
-"aio":{"master":{"change":"application","gzip7":{"inline":925,"main":119519,"polyfills":11863},"gzip9":{"inline":925,"main":119301,"polyfills":11861},"uncompressed":{"inline":1533,"main":486493,"polyfills":37068}}}
+  "aio": {
+    "master": {
+      "gzip7": {
+        "inline": 925,
+        "main": 119519,
+        "polyfills": 11863
+      },
+      "gzip9": {
+        "inline": 925,
+        "main": 119301,
+        "polyfills": 11861
+      },
+      "uncompressed": {
+        "inline": 1533,
+        "main": 486493,
+        "polyfills": 37068
+      }
+    }
+  }
 }

aio/scripts/payload.sh

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