radicle: Keep pinned repos ordered in config — heartwood

A trivial patch suggested by @Johannes in zulip:

Do not return them in a random order in radicle-httpd, so that we can have them in a defined order in radicle-explorer.

 #[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
 pub struct Pinned {
     /// Pinned repositories.
     pub repositories: HashSet<RepoId>,
     pub repositories: Vec<RepoId>,
 }