ci: install `bash5` on macOS via Brew w/ no auto-update

[agilgur5]

Summary

Use HOMEBREW_NO_AUTO_UPDATE=1 brew install bash to install bash5 via Brew and skip Brew's slow auto-update

Details

• per the deleted in-line comments, the reason it was installed as a static binary was because Brew's auto-update is slow
  • we can disable the auto-update though, so do that instead in order to install via Brew

Slowly making things a bit less hacky!

Testing

Can see CI passing on my branch here (which is built on top of #62 for CI to exist)