use env bash rather than /bin/bash

My bash is not installed in /bin/bash.

test/fast/Running "nvm use x" should create and change the "current" symlink

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 export NVM_SYMLINK_CURRENT=true
 . ../../nvm.sh

test/fast/Running "nvm use x" should not create the "current" symlink if $NVM_SYMLINK_CURRENT is false

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 . ../../nvm.sh