Update pgbackrest-initialization script to be more portable

/bin/bash doesn't work on systems like nix /usr/bin/env bash does
other scripts throughout the repo use the more portable option

Author: sfc-gh-jmckulka

testing/kuttl/scripts/pgbackrest-initialization.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 EXPECTED_STATUS=$1
 EXPECTED_NUM_BACKUPS=$2