Action Prettier fail

[Ekristoffe]

In the Workflow you have this line:
ref: ${{ github.head_ref }}
Which work fine if the pull request is not from a fork but as soon as the request came from a fork it will fail.
Could you use this instead:
ref: ${{ github.event.pull_request.head.ref }}
Which will work for Upstream and fork.

Ps It seem that if you use Checkout v2 this one work better:
ref: ${{ github.event.pull_request.head.sha }}

Thanks