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
b00a95b678