nixos: make initrd-network-ssh test a channel blocker

Last year the initrd sshd broke due to an openssh update which looked
innocent enough (the change that broke the initrd was mentioned in the
changelog, but you'd be forgiven for not making the connection):
- https://github.com/NixOS/nixpkgs/pull/323753
- https://github.com/NixOS/nixpkgs/pull/323796

Hopefully this won't happen again, the initrd test has been added to
passthru.tests for openssh since:
https://github.com/NixOS/nixpkgs/pull/356190

However, it is probably best to also have such an issue block the
channel. The ssh initrd is probably almost exclusively used on
remote machines where it is really bad when the initrd sshd
doesn't come up since it is used to unlock an encrypted volume
or similar, so it'd be stuck in initrd indefinitely. Also,
for such systems it is usually very difficult to impossible
to easily choose a different generation to boot into via
the boot loader.