no longer should be run under sudo

The script runs sudo itself now and should no longer be invoked under sudo. It does this to limit the amount of shell script that must be run as root. now is interactive, and asks for confirmation before continuing.

Installation is like:

$ curl -sSL | sh

Which will print:

Welcome to Rust.

This script will download the Rust compiler and its package manager, Cargo, and
install them to /usr/local. You may install elsewhere by running this script
with the --prefix=<path> option.

The installer will run under 'sudo' and may ask you for your password. If you do
not want the script to run 'sudo' then pass it the --disable-sudo flag.

You may uninstall later by running /usr/local/lib/rustlib/,
or by running this script again with the --uninstall flag.

Continue? (y/N)