scripts: Use /usr/bin/env bash instead of /bin/bash.

Same approach as is common for python.

Fixes builds on NixOS and other environments that guarantee
reproducible builds.

build

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 set -e
 

scripts/semaphore.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 set -e
 

test

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Test script code thanks to coreos/etcd
 #