How do I tell Homebrew to stop running brew update every time I want to install something?
Just prefix your install command with HOMEBREW_NO_AUTO_UPDATE=1, like this:
HOMEBREW_NO_AUTO_UPDATE=1 brew install somepackage
Source: brew manpage
I just modified /usr/local/bin/brew to add HOMEBREW_NO_AUTO_UPDATE=1 (according to @D Schlachter answer) at the start of the file