rustbuild: enable an initial local cargo

rustbuild: enable an initial local cargo

This allows the initial build of src/bootstrap itself to use a local
cargo taken from `configure --local-rust-root`.  It was already finding
rustc this way, but was always downloading cargo since it didn't know
where to find it.

It now matches the same logic that `config.rs` will use for stage0,
where both rustc and cargo are taken from `CFG_LOCAL_RUST_ROOT`.