don't mess up release note generation if there are multiple releases

[Gankra]

really we just don't handle multiple releases properly at all, but I'm pretty sure something funky will happen to the release note generation code/scripts if there are multiple. Possibly they'll end up concatenated? Or one will get picked arbitrarily?

Ideally you workflow either looks like:

• I am cutting a unified release for everything in the workspace, merge the release notes harmoniously
• I am cutting N logically independent releases at once, create N github releases with their own ntoes
• I am cutting a release for only one the projects in the workspace

The last 2 may actually end up looking the same, as you may just push N commits with tags like "specific-app-vX.Y.Z". This kind of pattern isn't at all handled by the CI scripts though (but I'd like it to be!). But if we're adding that kind of complex parsing/detection of the git tag, I'd like that logic to somehow live in cargo-dist itself (no complex logic in CI scripts!!!).

[Gankra]

working on a more holistic fix to this in #111