consistently use bash shebang for shell scripts

This updates the build scripts to use a bash shebang for consistency
with other scripts as well as better portability. If using a non-alpine
BUILDIMAGE (e.g. debian) the build scripts are not valid sh syntax.

build/build.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 #
 # Copyright 2016 The Kubernetes Authors.
 #

build/test.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 #
 # Copyright 2016 The Kubernetes Authors.
 #