SCRIPTS: use /usr/bin/env bash instead of /bin/bash for scripts

Given that some OSes have bash in /usr/local/bin and in order not to
give too easy an excuse to Olivier for not backporting fixes, let's
make a few scripts rely on /usr/bin/env bash instead of /bin/bash :-)

scripts/announce-release

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # prepares a template e-mail and HTML file to announce a new release
 # Copyright (c) 2006-2016 Willy Tarreau <w@1wt.eu>
 #

scripts/backport

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 USAGE="Usage: ${0##*/} <last> <commit> [...]"
 START="$PWD"

scripts/create-release

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # creates a new haproxy release at the current commit
 # Copyright (c) 2006-2016 Willy Tarreau <w@1wt.eu>
 #

scripts/git-show-backports

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #
 # Compares multiple branches against a reference and shows which ones contain
 # each commit, and the level of backports since the origin or its own ancestors.

scripts/publish-release

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # puts the public files online after a release
 # Copyright (c) 2006-2016 Willy Tarreau <w@1wt.eu>
 #