Favor GITHUB_WORKFLOW_REF

Introduced with GHES 3.9:
https://docs.github.com/en/enterprise-server@3.9/actions/learn-github-actions/variables

GITHUB_WORKFLOW_REF means that actions don't need to use `actions: read`
to determine the path to the running workflow.

Merge pull request github#1301 from github/aeisenberg/remove-set-output

Removes deprecated set-output usage

Merge pull request github#1301 from github/aeisenberg/remove-set-output

Removes deprecated set-output usage