Fixes #1959. This PR hash-pins GitHub Actions used in workflows and sets up dependabot to keep the Actions up-to-date. I've configured dependabot to group all Actions together. So if it ever discovers that multiple Actions have new versions, it'll only send a single PR to bump them all together. --------- Signed-off-by: Pedro Kaj Kjellerup Nacht <pnacht@google.com>