configs/releng/packages.x86_64: add open-iscsi
From https://wiki.archlinux.org/title/ISCSI/Boot#iSCSI_initiator_setup:
Install the open-iscsi package in the live installer.
Doing as it says can lead to a partial upgrade since updating all packages in the live environment is unfeasible.
Add open-iscsi
to configs/releng/packages.x86_64
to support this use case without such issues.