Include precompiled metadata in release files

[jonatanklosko]

We currently make an API request to fetch the available precompiled binaries. This may hit API rate limiting as in #52 (comment).

In the past, we would sometimes add more precompiled binaries after the release, however now we always build them upfront. Consequently, we can generate a metadata file as part of the release process that knows all the precompiled targets.