Automerge: [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: llvm/llvm-project#167434

Author: boomanaiden154