Continuing with the goals of making the installer:
- more useful
- entirely offline
Let's pre-download the API data from a `brew update` run and install it
into the logged-in user's home directory.
While we're here, in the `postinstall` script:
- use longer arguments for various commands
- fix an issue with symlinking on Intel if `/usr/local/bin` doesn't
already exist
- unset `bash -x` and use `-v` on more commands instead
- don't include GitHub Actions credentials
- generate and include RTF version of Homebrew BSD license
- create nicer installer with `productbuild`
- customise welcome/conclusion messages
- add Homebrew logo as a background image
- remove unnecessary `preinstall` script
- cleanup `postinstall` script to pass `brew doctor`
- use `Distribution.xml` for a nicer installer
- require CLT or Xcode rather than using our hacks to try and install
CLT during installation
