stand/loader.efi: fix regression with ignoring nvstore

To read/update the boot loader nvstore, we always need to call
zfs_attach_nvstore() regardless of whether we use bootonce key
in nvstore or the bootfs property of the pool. The call was
unintentionally left in the block of code that is processed
only when bootonce key is present.

In particular this fixes broken 'nextboot -k'.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D41795
Fixes: e3e2681d0ec28d6661fef6da76c9479049e2761c