Fix shebang for termux.

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

tests/grub_script_no_commands.in

@@ -1,4 +1,4 @@
-#! /bin/sh
+#! @BUILD_SHEBANG@
 set -e
 
 # grub-script-check refuses to pass a file with no commands; this usually