treewide: fix some shebangs

Some bash scripts have shebangs that point directly to the bash
interpreter, rather than going through /usr/bin/env. Replace these in
in bash scripts that are invoked in tests.

Signed-off-by: Mark Pashmfouroush <mark@isovalent.com>

bpf/tests/builtin_gen

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
 # Copyright Authors of Cilium
 

pkg/datapath/loader/check-sources.sh

@@ -1,4 +1,6 @@
-#! /bin/bash -efu
+#!/usr/bin/env bash
+
+set -efu
 
 #
 # This simple awk command extracts file names from the definition of the

test/provision/docker-run-cilium-docker-plugin.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 CILIUM_DOCKER_PLUGIN_IMAGE=${CILIUM_DOCKER_PLUGIN_IMAGE:-cilium/docker-plugin:latest}