.github: separate release creation from uploading assets

[ee7]

Before this commit, the .github/bin/publish-release script handled both:

• creating the draft release
• uploading assets to the release

with the intention that the first build job to finish would create the release with one asset, and each later job would upload an asset.

The logic was like:

if ! gh release view "${build_tag}"; then
  gh release create [...]
else
  gh release upload [...]
fi

But this had a race. For example, if two build jobs finished at nearly the same moment, they could each create a draft release. This race was almost never encountered in practice, and was easily resolved by deleting a draft release and restarting a build job, but it's worth resolving.

Add an initial build job to create a draft release that has no assets, and make the later build jobs only upload to that release.

This fixes the race, and makes it easier to share logic between the native build jobs and the cross-compiling build jobs.

Fixes: #551
Refs: #789

---

To-do:

• Test build job
• Follow-up PR: rename build scripts (do separately to ensure that git can track the rename)
• Follow-up PR: make cross-compilation job run in parallel with native build jobs

[ee7]

I manually triggered this PR's build workflow. It worked, and created this draft release.

CI indicates failure due to rate limiting in the macos fetch-configlet job. Edit: It worked after some restarts.