nixos/tests/installer: fix clevis ZFS tests

Follow-up to c5feb3c424 / #525613, which fixed zfsroot and
separateBootZfs after a2e55e31d6 flipped the default of
boot.zfs.forceImportRoot to false, but missed the clevis ZFS tests.
Without a matching hostId on the installer side, the target refuses to
import the pool ("pool was previously in use from another system").

nixos/tests/installer.nix

@@ -1118,6 +1118,7 @@ let
         enableOCR = fallback;
         extraInstallerConfig = {
           boot.supportedFilesystems = [ "zfs" ];
+          networking.hostId = "00000000";
           environment.systemPackages = with pkgs; [ clevis ];
         };
         createPartitions = ''