Performance Tests Workflow: Polish Bash script

[mcsf]

Description

This PR consists of three commits:

Avoid non-POSIX grep -P

grep -P is a GNU extension to use PCRE regular expressions. While it is commonly available on GNU/Linux systems, it is not POSIX and cannot be guaranteed to exist. By replacing it with POSIX-compliant AWK, this workflow is easier to test locally on different systems, including all BSD-derived ones.

Use safer or more modern Bash

• Prefer $(( ... )) over antiquated expr
• Use double quotes to prevent globbing and word splitting
• Condense read options... just because

As a general note, Shellcheck is an excellent tool for linting and validating shell scripts.

Explicitly require Bash shell

The run block for the "Compare performance with current WordPress Core and previous Gutenberg versions" job relies on a series of Bashisms (e.g. <<< syntax, arrays), so we should make that an explicit requirement.

Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).