Before this, `sudo brew cask` will auto install Homebrew cask as root
user. This will cause permission problem for files in `Tap` directory.

Therefore, let's check process uid and switch to normal user for tap.
As result, `sudo brew cask` will work as the same before, except tap
files will have the correct permission attributes.

Closes Homebrew/homebrew#48059.

Signed-off-by: Xu Cheng <xucheng@me.com>