abuild: change shebang to /bin/ash

This script uses variable expansion / pattern replacement, which is not
defined in POSIX-sh. Also APKBUILDs are not required to be strictly
POSIX-sh compatible. If someone run abuild on system with e.g. dash as
/bin/sh, then it fails (and someone reported exactly that on IRC
today). Therefore abuild should explicitly use /bin/ash in shebang and
not /bin/sh.

abuild.in

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/ash
 
 # abuild - build apk packages (light version of makepkg)
 # Copyright (c) 2008-2015 Natanael Copa <ncopa@alpinelinux.org>