testsuite: Fix hardcoded /bin/bash in custom-cc wrapper

Not all operating systems have `/bin/bash` available, it is more portable
to use `/usr/bin/env bash`

For example: https://stackoverflow.com/questions/21612980/why-is-usr-bin-env-bash-superior-to-bin-bash

cabal-testsuite/PackageTests/CCompilerOverride/custom-cc

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 if which cc >/dev/null 2>&1; then
     cc -DNOERROR6 "${@}"