brew: tighten sudo checks

`reinstall` can currently be executed under sudo and has every chunk of problems
running install under sudo does when `brew` isn't root owned.

Likewise with `postinstall` which we increasingly use to create directories and
move/install files elsewhere. Using sudo there both masks deeper issues and
makes uninstall troublesome.

Closes Homebrew/homebrew#45112.

Signed-off-by: Dominyk Tiller <dominyktiller@gmail.com>