For beta (at least, but I assume the same for nightly), the build system launchs `git` commands for retreiving the beta version. The `beta_prerelease_version()` function was introduced by https://github.com/rust-lang/rust/commit/06913a5b1d19284ae2c2ff6cfac130842334e886 . Now, the build just fail as git command fail (tarballs aren't git repository).