homebrew更新慢换源

更换为中科大源：

#1.替换brew.git:
cd "$(brew --repo)"
git remote set-url origin https://mirrors.ustc.edu.cn/brew.git

#2.替换homebrew-core.git:
cd "$(brew --repo)/Library/Taps/homebrew/homebrew-core"
git remote set-url origin https://mirrors.ustc.edu.cn/homebrew-core.git

#3.替换homebrew-cask.git:
cd "$(brew --repo)"/Library/Taps/homebrew/homebrew-cask
git remote set-url origin https://mirrors.ustc.edu.cn/homebrew-cask.git

#4.更新:
brew update