rust-installer: Use env(1) in the shebang.

This fixes the case (e.g. *BSD) where bash is installed on the host
system, but not at the typical location of /bin.

src/tools/rust-installer/install-template.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # No undefined variables
 set -u