`release.json` doesn't have a trailing newline in 10.0-rc.2

[corngood]

The release.json now doesn't have a trailing newline, which blows up the nixpkgs linter. Every other one up to this point seems to have had it.

Originally posted by @corngood in #5367 (comment)

This is not difficult to work around, but it's generally considered an error by git, etc.

[ellahathaway]

Fixed in this commit. Also added testing for this so there won't be a regression in the future.