update shebang to reflect bashisms

Update shebang, given that `local` is not a POSIX semantic.

rustup-init.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 # Copyright 2016 The Rust Project Developers. See the COPYRIGHT
 # file at the top-level directory of this distribution and at
 # http://rust-lang.org/COPYRIGHT.