Split r+ from retry permissions

[edunham]

In Rust, we'd like to be able to give some users the ability to retry a build without the permission to r+ PRs.

[barosl]

I guess this is already possible by the try_users feature (which is introduced by #104)? The Homu version Rust's using is very old. As Homu has had some breaking changes, a manual upgrade would be needed.