nixos/release-combined.nix: Make 32-bit iso_minimal a channel blocker

In practice it already is since the channel update script barfs out if
the job fails:

https://github.com/NixOS/nixos-channel-scripts/blob/9f96e1e25d197de46d5dbe3c4f81b8d57b9c9821/mirror-nixos-branch.pl#L132