bootstrap: avoid ever invoking mkNixpkgs without the localSystem argument

apparently importing nixpkgs and then `override`ing with `localSystem`
isn't enough; it needs `localSystem` from the start.

impure.nix

@@ -9,7 +9,7 @@
 }:
 let
   mkPkgs = branch: args: (
-    (import ./pkgs/by-name/nixpkgs-bootstrap/${branch}.nix {}).override args
+    import ./pkgs/by-name/nixpkgs-bootstrap/${branch}.nix args
   ).extend (import ./overlays/all.nix);
   pkgs = mkPkgs "master" { inherit localSystem; };
   inherit (pkgs) lib;

pkgs/by-name/nixpkgs-bootstrap/master.nix

@@ -5,11 +5,12 @@
 # - nix hash to-sri sha256:xyz
 #   - paste the output as the new `sha256`
 {
-  mkNixpkgs ? import ./mkNixpkgs.nix {}
+  mkNixpkgs ? import ./mkNixpkgs.nix {},
-}:
+  ...
-mkNixpkgs {
+}@args:
+mkNixpkgs ({
   rev = "5db39d669a21ca7f0e76c295e7664b9d938b7042";
   sha256 = "sha256-SUZZMAn19X3Ym6lVb18LoXhCS2yicrRr83si5VyJDS0=";
   version = "0-unstable-2025-02-28";
   branch = "master";
-}
+} // args)

pkgs/by-name/nixpkgs-bootstrap/staging-next.nix

@@ -1,9 +1,10 @@
 {
-  mkNixpkgs ? import ./mkNixpkgs.nix {}
+  mkNixpkgs ? import ./mkNixpkgs.nix {},
-}:
+  ...
-mkNixpkgs {
+}@args:
+mkNixpkgs ({
   rev = "749375426d72ead4bdac625818e7be62a6bbbaf4";
   sha256 = "sha256-IDxPfbSdIy7XAP1hneGOfr2jsj+hFUsvFhpRksYqols=";
   version = "0-unstable-2025-02-28";
   branch = "staging-next";
-}
+} // args)

pkgs/by-name/nixpkgs-bootstrap/staging.nix

@@ -1,9 +1,10 @@
 {
-  mkNixpkgs ? import ./mkNixpkgs.nix {}
+  mkNixpkgs ? import ./mkNixpkgs.nix {},
-}:
+  ...
-mkNixpkgs {
+}@args:
+mkNixpkgs ({
   rev = "29dcbf482396b9e5bdf1ec92973a8451e0aaa1d5";
   sha256 = "sha256-ps1xz98RAUqrT+V7GFpzf/uHaoh9o5ZoOpE7SnSB6sY=";
   version = "0-unstable-2025-02-28";
   branch = "staging";
-}
+} // args)