Improve GitHub Releases (description, extra assets)

[jimeh]

Some of the things I'd like to add to each release:

• [ ] Summary of what was built, potentially the plan.yml file contains everything needed.
• [ ] Arguments passed to both ./configure and make. This will require changes in the build script.
• [ ] Capture ./configure output and upload it as a release asset.
• [ ] Add a code block to description with SHA256 sums for all attached assets. Potentially generated by simply combining the content of each *.sha256 asset.
• [ ] Upload the Emacs source tarball downloaded by the build script as a asset on each release.

For now these should all be relatively easy to deal with as each release will only have a single build for macOS 10.15 on x86_64. Once we get another OS or architecture the process will need to be able to deal with two concurrent builds attaching different assets and updating relevant parts of the description.