Update shebangs for environmentally finding Bash -- Compatibility update to...

Update shebangs for environmentally finding Bash -- Compatibility update to provide support for platforms that do not ship Bash in their /bin (#2841)

.presubmit/context

-#!/bin/bash
+#!/usr/bin/env bash
 
 echo "** presubmit/$(basename $0)"
 

.presubmit/filename-hyphen

-#!/bin/bash
+#!/usr/bin/env bash
 
 echo "** presubmit/$(basename $0)"
 

.presubmit/import-testing

-#!/bin/bash
+#!/usr/bin/env bash
 
 echo "** presubmit/$(basename $0)"
 

.presubmit/test-lowercase

-#!/bin/bash
+#!/usr/bin/env bash
 
 echo "** presubmit/$(basename $0)"
 

.presubmit/trailing-whitespace

-#!/bin/bash
+#!/usr/bin/env bash
 
 echo "** presubmit/$(basename $0)"