Automerge: [Github] Make unprivileged-download-artifact download multiple artifacts

This is designed to allow a workflow (e.g., premerge) upload comments
across multiple jobs. Subsequent PRs will wire this up within the
issue-write workflow to support reading comments from multiple files.

Reviewers: tstellar, cmtice

Reviewed By: cmtice

Pull Request: llvm/llvm-project#170216

Author: boomanaiden154