buildClan function: export all machines via nixosModules/darwinModules

We want each machine not only to be exposed via nixosConfigurations but also as a module.
This allows re-importing the machine in tests and override the architecture for example.

templates/clan/minimal/flake.nix

@@ -10,6 +10,6 @@
     in
     {
       # all machines managed by Clan
-      inherit (clan) nixosConfigurations clanInternals;
+      inherit (clan) nixosConfigurations nixosModules clanInternals;
     };
 }