我已经安装了自制软件，但是颤动说它没有安装：
iOS toolchain - develop for iOS devices (Xcode 9.3)
• Xcode at /Applications/Xcode.app/Contents/Developer
• Xcode 9.3, Build version 9E145
✗ Brew not installed; use this to install tools for iOS device development.
Download brew at https://brew.sh/.
brew --version打印
Homebrew 1.6.3
Homebrew/homebrew-core (git revision 27e7; last commit 2018-05-11)
我做错了什么？