Fix shebang for termux.

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

grub-core/modinfo.sh.in

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!@BUILD_SHEBANG@
 
 # User-controllable options
 grub_modinfo_target_cpu=@target_cpu@