Fix shebang for termux.

Termux doesn't have a /bin/sh. So we needto use $SHELL.
Keep /bin/sh as much as possible.

tests/cdboot_test.in

@@ -1,4 +1,4 @@
-#! /bin/sh
+#! @BUILD_SHEBANG@
 # Copyright (C) 2013  Free Software Foundation, Inc.
 #
 # GRUB is free software: you can redistribute it and/or modify