Publish a nightly prerelease with an installable build per host

Per-commit preview builds cover a pull request under review. They do not give anyone a
current build of the default branch to pick up and try.

A scheduled job should build an installable artifact for each host from the default branch,
smoke test it before publishing, verify every expected asset is present and non-empty, and
publish them together as a prerelease so they are easy to find and grab.

Steps

• Build an installable artifact per host from the default branch
• Smoke test each artifact before it is published
• Verify every expected asset exists and is non-empty
• Publish as a prerelease under a predictable tag

Done when

Anyone can install last night's build of any host without a toolchain or a pipeline run.