Make pod install a long-running step. (#8999)

The very first time `pod install` is invoked, CocoaPods downloads the master spec repository, which takes quite a while. Before this change, the build appeared to have stalled. With this change, at least the spinner is moving.

Added `pod setup` to the install instructions for CocoaPods, so the spec repo is downloaded while setting up Flutter, instead of during the first build.