`mkFlake`: limit nixosConfiguration flake check definitions to hosts defined and managed by `digga`

[tomeon]

I have a digga-managed flake where I do something like the following:

{
   inputs = { 
     # <snip>
   };

  outputs = { digga, ... }:
    let
      flake = digga.lib.mkFlake {
        # <snip>
      };

      extendedNixosConfigurations = lib.mapAttrs' (name: value: {
        name = "${name}-extended";
        value = value.extendModules {
          modules = [ 
            # <snip>
          ];
        };
      }) flake.nixosConfigurations;
    in 
    flake 
    // 
    {
      nixosConfigurations = flake.nixosConfigurations // extendedNixosConfigurations;
    };
}

Running nix flake check, I get this error:

error: attribute 'somehost-extended' missing

       at /nix/store/gyv51hksh3bngdqvafrwil6liskb57c1-source/src/mkFlake/outputs-builder.nix:131:42:

          130|             in
          131|             builtins.listToAttrs (map op config.nixos.hosts.${n}.tests);
             |                                          ^
          132|

This is because the outputs-builder.nix code that defines nixosConfiguration-related flake checks assumes that all entries in nixosConfigurations have corresponding entries in config.nixos.hosts. This PR updates said code to define checks only for hosts that digga manages, filtering out whatever other hosts the user may have added to the nixosConfigurations flake output.

Thank you for your consideration!

---

Side note: am I just holding it wrong? Maybe there's a more ergonomic way to extend previously-defined NixOS configurations -- perhaps something in the mkFlake interface that I've managed to overlook? Would be cool if something like this were possible:

{
  flake = digga.lib.mkFlake {
    # <snip>

    extend = final: prev: {
      nixosConfigurations = prev.nixosConfigurations // lib.mapAttrs' (name: value: {
        name = "${name}-extended";
        value = value.extendModules {
          modules = [ 
            # <snip>
          ];
        };
      }) prev.nixosConfigurations;
    };
  };
}