Based on the existing npm task, I created a new task to execute yarn to install packages in the build.
It also has a demand on the task.json requiring yarn, so it won't work on hosted agents, althought it would be a little non-sense, as one of the interesting features in Yarn is the local cache to make package installation and download faster, which will have no effect in a newly provisioned agent per each build.
The usage is pretty straight forward, working as the npm task:
