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

nixos/release-combined.nix

···
         (all nixos.dummy)
         (all nixos.manual)
 
-        (all nixos.iso_minimal)
+        nixos.iso_minimal.x86_64-linux
+        nixos.iso_minimal.i686-linux
         nixos.iso_graphical.x86_64-linux
         nixos.ova.x86_64-linux