[Github] Add Workflow to test Unprivileged Download Artifact Action
Since this is a composite action, we do not get any testing currently
when updating the action. This patch adds a simple workflow to test the
action so we can ensure we do not break it when modifying it.
Reviewers: tstellar
Reviewed By: tstellar
Pull Request: https://github.com/llvm/llvm-project/pull/167434