Initial PR-based artifact update workflow

[omus]

Working revising the workflow for updating this package to reflect new tzdata releases. The plan for this is:

1. Have a cron triggered workflow detects new tzdata versio, build the tarball, and updates the Project.toml and Artifacts.toml. The tarball is uploaded as a GitHub actions artifact. Then a PR is created with these changes.
2. The CI tests in the PR detect that the Artifact.toml has changed and download the action artifact. A ~/.julia/artifacts/Overrides.toml is created based upon the Artifacts.toml content. This allows the CI to pass.
3. When the PR is merged the actions artifact is downloaded and then a GitHub release is created and the release artifacts are uploaded
4. The new release is registered

[omus]

I've attempted to test as much of this as possible with pull_request events. I've most likely overlooked something but I'm willing to merge this and try it out for real.