Minor fixes, cleanups, and tweaks

[kylewlacy]

This PR makes a few tweaks to the action:

• Improve install-brioche.sh
  • Fix issue where using $HOME would error out before checks around $HOME (due to set -u shell option)
  • Take inputs via env vars to prevent potential shell escaping issues
  • Manually validate and expand env vars in install-dir using envsubst (this is only done if install-dir looks like it could contain an env var)
  • Add install-dir to $GITHUB_PATH directly in the shell script, instead of in a separate action step
  • Add the Brioche package installation path to $GITHUB_PATH as well, so installed packages are available
• Update test workflow to install and use the hello_world package. This validates that installed packages are available after running the action
• Update README
  • Various minor rewordings
  • Improve example workflow (fix brioche usage, switch to actions/checkout@v4)