make-disk-image: run tune2fs after umount to skip fsck

tune2fs marks the filesystem as clean to prevent resize2fs from
complaining.

But we were invoking it before we mounted the filesystem, so the
counters would increase to 1 and it broke the functionality.

By moving the call after the mount, I have confirmed it works by:

   $ nix-build nixos/tests/ec2.nix

cc @rbvermaa @edolstra