Teach rustup.sh about release channels

[brson]

Right now it just pulls down the nightly tarball directly, which it knows by name. To work with arbitrary release channels it needs to download the manifest (e.g. 'channel-rust-nightly') and pull the correct installer name from it. The artifacts will not always have predictable names, particularly on the stable channel. Add a '--channel=' flag to rustup.sh.

[brson]

I'll probably do this in the next few days along with the other work I'm doing on the installer.

[steveklabnik]

@brson did this get done? I forget.

[richo]

Are the mechanics for where to find the artifacts documented somewhere? Adding the flag is easy enough, I can finish this up if pointed to where the files live.

[caipre]

@steveklabnik Looks like this should be closed: https://github.com/rust-lang/rustup/blob/404d1429dbb6aa46daf92ebf268c24a93474e136/rustup.sh#L1321