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.