Replace #!/bin/bash shebangs with #!/usr/bin/env bash

For portability. E.g. /bin/bash doesn't exist on NixOS.

src/timeprivacy

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 ## Copyright (c) 2013, adrelanos at riseup dot net
 ## All rights reserved.

test/test_variable_data.sh

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

test/testframe.sh

@@ -1,4 +1,4 @@
-#! /bin/bash
+#!/usr/bin/env bash
 #	testframe.sh DIR
 # bare-bones testing framework.
 # run the test suites in the given DIR;