flake.nix: Drop systemAgnosticOutputs.nixosConfigurations

There was a TODO about removing them, and it caused issues now when we evaluated into them
and got to parsing `system = null`.

`makemake` has been moved into `eachDefaultSystemOutputs.nixosConfigurations`.
Nixpkgs was then not happy with the way we were importing the system configuration for `makemake`,
so adjust the entry point of our import until it's happy.

- Errored out on us importing an entire config into the module system, so change it into a module
- sops-nix was getting imported twice (it's in `default.nix`'s `extendedNixosModules` already),
  so drop the second import from `makemake`.

Author: OPNA2608

flake.nix

@@ -102,12 +102,6 @@
 
       # Finally, define the system-agnostic outputs.
       systemAgnosticOutputs = {
-        nixosConfigurations =
-          # TODO: remove these, noone will (or can even, realistically) use them
-          mapAttrs (_: mkNixosSystem classic') rawExamples // {
-            makemake = import ./infra/makemake { inherit inputs; };
-          };
-
         inherit nixosModules;
 
         # Overlays a package set (e.g. Nixpkgs) with the packages defined in this flake.
@@ -141,6 +135,10 @@
           };
         in
         rec {
+          nixosConfigurations = mapAttrs (_: mkNixosSystem classic) {
+            makemake = import ./infra/makemake { inherit inputs; };
+          };
+
           packages = ngipkgs // {
             overview = import ./overview {
               inherit lib lib' self;
@@ -212,7 +210,7 @@
                     nixfmt-rfc-style.enable = true;
                   };
                 };
-                "infra/makemake" = toplevel self.nixosConfigurations.makemake;
+                "infra/makemake" = toplevel self.nixosConfigurations.${system}.makemake;
                 "infra/overview" = self.packages.${system}.overview;
                 # fake derivation for flake check
                 "infra/templates" = pkgs.writeText "dummy" (lib.strings.toJSON classic'.templates.project);

infra/makemake/default.nix

@@ -1,12 +1,9 @@
 { inputs }:
-inputs.nixpkgs.lib.nixosSystem {
-  system = "x86_64-linux";
-
-  modules = [
+{
+  imports = [
     # Setup both a master and a worker buildbot instance in this host
     inputs.buildbot-nix.nixosModules.buildbot-master
     inputs.buildbot-nix.nixosModules.buildbot-worker
-    inputs.sops-nix.nixosModules.default
     ./configuration.nix
     { nix.nixPath = [ "nixpkgs=${inputs.nixpkgs}" ]; }
   ];