Upgrade `bazel-skylib` to `1.4.0` and regenerate `README`

What is the goal of this PR?

We upgraded bazel-skylib to 1.4.0 and regenerated the README file.

What are the changes implemented in this PR?

Recently GitHub was affected by a major incident where commit-based archives changed their SHAs, crippling our whole build infrastructure. bazel-skylib was one such affected package.

As a precaution, we've upgraded it to the latest version. This has also resulted in some incidental but helpful bugfixes in our generated README.md, which we've committed.

Also, the Packer OSX SHA256 hash was out of date, presumably due to Hashicorp regenerating their distribution bundle. We've fixed it and verified it against their provided document of expected hashes.

PR Review Checklist

Do not edit the content of this comment. The PR reviewer should simply update this comment by ticking each review item below, as they get completed.

Trivial Change

[x] This change is trivial and does not require a code or architecture review.

Code

[ ] Packages, classes, and methods have a single domain of responsibility.
[ ] Packages, classes, and methods are grouped into cohesive and consistent domain model.
[ ] The code is canonical and the minimum required to achieve the goal.
[ ] Modules, libraries, and APIs are easy to use, robust (foolproof and not errorprone), and tested.
[ ] Logic and naming has clear narrative that communicates the accurate intent and responsibility of each module (e.g. method, class, etc.).
[ ] The code is algorithmically efficient and scalable for the whole application.

Architecture

[ ] Any required refactoring is completed, and the architecture does not introduce technical debt incidentally.
[ ] Any required build and release automations are updated and/or implemented.
[ ] Any new components follows a consistent style with respect to the pre-existing codebase.
[ ] The architecture intuitively reflects the application domain, and is easy to understand.
[ ] The architecture has a well-defined hierarchy of encapsulated components.
[ ] The architecture is extensible and scalable.

vaticle / bazel-distribution