rust-installer needs updating

[mrmonday]

The version of rust-installer which is currently in the repository has a sub-module which is fetched over HTTP rather than HTTPS. This is insecure, and also breaks any GitHub Page which uses Rust as a sub-module, such as BitRust (https://github.com/mrmonday/bitrust, http://bitrust.octarineparrot.com/).

[alexcrichton]

cc @brson

[mrmonday]

Pull #23763 seems to have fixed this.