Make sure rustup.sh is safe from truncation errors

[erickt]

Someone on hacker news pointed out that bad things could potentially happen if the running of curl https://static.rust-lang.org/rust.sh | sh gets truncated on a bad line. We should audit the code to make sure that can't happen. Maybe we could write the actual rustup.sh and verify it with a checksum before we execute it?

[huonw]

This would theoretically be resolved by putting the whole script into a function that then gets executed at the very end, so nothing happens until that is reached, e.g.

f() {
    rm -Rf "${TMP_DIR}"
    need_ok "failed to remove temporary installation directory"

    mkdir -p "${TMP_DIR}"
    need_ok "failed to create create temporary installation directory"

    ...
    rm -Rf "${TMP_DIR}"
    need_ok "couldn't rm temporary installation directory"
}

f

(Tagging as E-easy due to this, I believe the file is available in src/etc.)