Generates the checksum file the hex package ships.
Downloads every per-target forcola_shim tarball for the current
mix.exs version from GitHub Releases, computes each SHA256 locally,
and writes checksum-forcola_shim.exs (a map of artifact
name to hex digest).
Run after the release workflow for the tag has finished and before
mix hex.build / mix hex.publish. The file is gitignored; it exists
only in the published package, which is also what switches consumer
compiles onto the download-and-verify path.