testsuite: Use /usr/bin/env instead of /bin/bash

Namely in T13719 and T13701.

testsuite/tests/perf/compiler/genT13701

-#!/bin/bash
+#!/usr/bin/env bash
+
 # Generate $DEPTH layers of modules with $WIDTH modules on each layer
 # Every module on layer N imports all the modules on layer N-1
 # MultiLayerModules.hs imports all the modules from the last layer

testsuite/tests/perf/compiler/genT13719

-#!/bin/bash
+#!/usr/bin/env bash
+
 # Generate $DEPTH layers of modules with $WIDTH modules on each layer
 # Every module on layer N imports all the modules on layer N-1
 # $ROOT.hs imports all the modules from the last layer