mxgrub: Fix check if GRUB is installed on ESP

The condition checked for a directory, but it’s a file, and therefore
GRUB was always installed anew on the ESP. With the fixed condition it
works now.

    $ sudo mxgrub mariux-5.12.0-381
    mount -L ESP /boot/efi
    GRUB for UEFI is already installed.
    umount /boot/efi
    GRUB is already installed in MBR.
    grub-editenv - set chosen=all-other-kernel>mariux-5.12.0-381

Resolves: #180

mxgrub/mxgrub

@@ -325,7 +325,7 @@ sub cmd_install {
 
 	if (has_esp) {
 		sys 'mount','-L','ESP','/boot/efi';
-		if (-d '/boot/efi/EFI/grub/grubx64.efi') {
+		if (-e '/boot/efi/EFI/grub/grubx64.efi') {
 			print "GRUB for UEFI is already installed.\n";
 		}
 		else {