This improves CI security by ensuring that action dependencies cannot be
changed by upstream repositories without updating the use of the
dependency in this repo.
Official GitHub-maintained actions are excluded from this requirement
and are left pinned by a tag instead of a commit hash. This action
dependency pinning policy is codified in `.github/zizmor.yml` so that
Zizmor (a static analysis tool for GitHub Actions) doesn't flag these as
unpinned dependencies.
Also add cooldown timer for Dependabot. This fixes these two Zizmor
audits:
* https://docs.zizmor.sh/audits/#dependabot-cooldown
* https://docs.zizmor.sh/audits/#unpinned-uses
