dissect-image: port mount_image_privately_interactively() to use /run/systemd/mount-rootfs/ too

Let's use the same common directory as the unit logic uses.

This means we have less to clean up, and opens the door to eventually
allow unprivileged operation of the
mount_image_privately_interactively() logic.

src/sysupdate/sysupdate.c

@@ -861,7 +861,7 @@ static int process_image(
                 LoopDevice **ret_loop_device) {
 
         _cleanup_(loop_device_unrefp) LoopDevice *loop_device = NULL;
-        _cleanup_(umount_and_rmdir_and_freep) char *mounted_dir = NULL;
+        _cleanup_(umount_and_freep) char *mounted_dir = NULL;
         int r;
 
         assert(ret_mounted_dir);