Revert "clan: Remove pkgsForSystem"

This reverts commit 382e27a20b.

lib/build-clan/interface.nix

@@ -3,7 +3,6 @@ let
   types = lib.types;
 in
 {
-
   options = {
     # Required options
     directory = lib.mkOption {
@@ -70,6 +69,18 @@ in
       default = { };
     };
 
+    pkgsForSystem = lib.mkOption {
+      type = types.functionTo (types.nullOr types.attrs);
+      default = _: null;
+      defaultText = "Lambda :: String -> { ... } | null";
+      description = ''
+        A function that maps from architecture to pkg. `( string -> pkgs )`
+
+        If specified this nixpkgs will be only imported once for each system.
+        This improves performance, but all nipxkgs.* options will be ignored.
+      '';
+    };
+
     # Outputs
     nixosConfigurations = lib.mkOption {
       # Hide from documentation.
@@ -101,5 +112,4 @@ in
       };
     };
   };
-
 }