Use /usr/bin/env bash in Makefiles and scripts

This allows the execution of the Makefiles from distributions which do
have `bash` in a different path.

Signed-off-by: Sascha Grunert <sgrunert@suse.com>

build/root/Makefile

@@ -30,7 +30,7 @@ endif
 #   clean: Clean up.
 
 # It's necessary to set this because some environments don't link sh -> bash.
-SHELL := /bin/bash
+SHELL := /usr/bin/env bash
 
 # We don't need make's built-in rules.
 MAKEFLAGS += --no-builtin-rules

build/root/Makefile.generated_files

@@ -30,7 +30,7 @@ endif
 
 
 # It's necessary to set this because some environments don't link sh -> bash.
-SHELL := /bin/bash
+SHELL := /usr/bin/env bash
 
 # This rule collects all the generated file sets into a single rule.  Other
 # rules should depend on this to ensure generated files are rebuilt.

hack/update-workspace-mirror.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Copyright 2018 The Kubernetes Authors.
 #