Yarn is not installed

We are using a self-hosted runner and using this action to setup-node and install yarn.

I know on github-hosted runners, yarn is already pre-installed but it would be good if yarn was installed by this action for those of us using self-hosted.

Running setup-node with the following workflow and it works

 with:
    node-version: 12.13.0
    always-auth: true
    registry-url: https://registry.npmjs.org

/usr/bin/tar xz --warning=no-unknown-keyword -C /actions-runner/_work/_temp/febce1c0-314e-4a14-9a65-5bde8a990b71 -f /actions-runner/_work/_temp/d4bf33ab-571e-458e-9495-bd6c5b994814
/actions-runner/_work/_tool/node/12.13.0/x64/bin/node --version
v12.13.0
/actions-runner/_work/_tool/node/12.13.0/x64/bin/npm --version
6.12.0

Note that node and npm are installed.

A later step in the workflow then attempts to run yarn:

Run yarn install --ignore-scripts --frozen-lockfile
/actions-runner/_work/_temp/58e53b82-f9ae-4c68-9cec-75f75831208b.sh: line 1: yarn: command not found
##[error]Process completed with exit code 127.

This of course works on github hosted runners just fine, but since I just migrated to an EC2 instance that doesn't have yarn installed, I am now having issues.

Start by tracing the setup-node action entry point and how it installs Node and npm on self-hosted runners. The issue names no files or tests, so inspect the repository structure first. Done means a self-hosted runner can run yarn after setup-node, while the existing Node setup continues to work.