Minor cleanups to CONTRIBUTING.md (#8043)

This is really just in order to trigger a build. ci.chromium.org infra
had a short outage during our last build.

CONTRIBUTING.md

 Contributing to the Flutter engine
 ==================================
 
-[![Build Status](https://cirrus-ci.com/flutter/engine.svg)](https://cirrus-ci.com/flutter/engine)
+[![Build Status](https://cirrus-ci.com/flutter/engine.svg)][build_status]
 
-_See also: [Flutter's code of conduct](https://github.com/flutter/flutter/blob/master/CODE_OF_CONDUCT.md)_
+_See also: [Flutter's code of conduct][code_of_conduct]_
 
 Welcome
 -------
 
-For an introduction to contributing to Flutter, see [our
-contributor
-guide](https://github.com/flutter/flutter/blob/master/CONTRIBUTING.md).
+For an introduction to contributing to Flutter, see [our contributor
+guide][contrib_guide].
 
-For specific instructions regarding building Flutter's engine, see
-[Setting up the Engine development
-environment](https://github.com/flutter/flutter/wiki/Setting-up-the-Engine-development-environment)
-on our wiki. Those instructions are part of the broader onboarding instructions described in the contributing guide.
+For specific instructions regarding building Flutter's engine, see [Setting up
+the Engine development environment][engine_dev_setup] on our wiki. Those
+instructions are part of the broader onboarding instructions described in the
+contributing guide.
+
+[build_status]: https://cirrus-ci.com/flutter/engine
+[code_of_conduct]: https://github.com/flutter/flutter/blob/master/CODE_OF_CONDUCT.md
+[contrib_guide]: https://github.com/flutter/flutter/wiki/Setting-up-the-Engine-development-environment
+[engine_dev_setup]: https://github.com/flutter/flutter/wiki/Setting-up-the-Engine-development-environment