Teach rustup to download manifests from the `/staging/` directory
Teach rustup to download manifests from the `/staging/` directory

With the changes to `RUSTUP_DIST_SERVER` to automitally replace URLS
in the manifest, it's no longer usable to test Rust pre-releases!

The `RUSTUP_STAGED_MANIFEST` flag tells rustup to download *just* the
manifest from the `/staging/` directory on s.rlo.

theindigamer/rustup.rs,Boddlnagg/rustup.rs,polonez/rustup.rs,polonez/rustup.rs,rust-lang/rustup,theindigamer/rustup.rs,inejge/rustup.rs,theindigamer/rustup.rs,rust-lang/rustup,rust-lang/rustup,nodakai/rustup.rs,nodakai/rustup.rs,nodakai/rustup.rs,inejge/rustup.rs,rust-lang/rustup,Boddlnagg/rustup.rs,polonez/rustup.rs,nodakai/rustup.rs,nodakai/rustup.rs,theindigamer/rustup.rs,Boddlnagg/rustup.rs,polonez/rustup.rs,rust-lang/rustup,inejge/rustup.rs,theindigamer/rustup.rs,Boddlnagg/rustup.rs,inejge/rustup.rs,inejge/rustup.rs,polonez/rustup.rs,Boddlnagg/rustup.rs