Point the Cirrus badge to the master status, not everything including PRs (#20487)

README.md

 # <img src="https://flutter.io/images/flutter-mark-square-100.png" alt="Flutter" width="40" height="40" /> Flutter [![Join Gitter Chat Channel -](https://badges.gitter.im/flutter/flutter.svg)](https://gitter.im/flutter/flutter?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
-[![Build Status - Cirrus](https://api.cirrus-ci.com/github/flutter/flutter.svg)](https://cirrus-ci.com/github/flutter/flutter)
+[![Build Status - Cirrus](https://api.cirrus-ci.com/github/flutter/flutter.svg)](https://cirrus-ci.com/github/flutter/flutter/master)
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 A new mobile app SDK to help developers and designers build modern mobile apps for iOS and Android. Flutter is an open-source project currently in beta.