Fix shebang for termux.

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

tests/util/grub-shell-tester.in

@@ -1,4 +1,4 @@
-#! /bin/sh
+#! @BUILD_SHEBANG@
 set -e
 
 # Compares GRUB script output with BASH output.

tests/util/grub-shell.in

@@ -1,4 +1,4 @@
-#! /bin/sh
+#! @BUILD_SHEBANG@
 set -e
 
 # Run GRUB script in a Qemu instance