Fix scripted installs on EFI systems using ZFS root with zfsboot.

Unlike attended installations, scripted installs did not mount non-ZFS
partitions when ZFS root (via zfsboot) was selected. Since this included
the ESP, the EFI loader was not installed. Copy logic from the
attended-install path to make this work.

PR:		255824, 255081
MFC after:	1 week
Obtained from:	Mark Huizer

usr.sbin/bsdinstall/scripts/script

@@ -111,8 +111,8 @@ if [ "$ZFSBOOT_DISKS" ]; then
 	bsdinstall zfsboot
 else
 	bsdinstall scriptedpart "$PARTITIONS"
-	bsdinstall mount
 fi
+bsdinstall mount
 
 # Unpack distributions
 bsdinstall checksum
@@ -163,9 +163,8 @@ fi
 bsdinstall entropy
 if [ "$ZFSBOOT_DISKS" ]; then
 	zpool export $ZFSBOOT_POOL_NAME
-else
-	bsdinstall umount
 fi
+bsdinstall umount
 
 f_dprintf "Installation Completed at %s" "$( date )"