flake: apply nixpatches/list.nix using the build patcher, not the emulated host patcher

flake.nix

@@ -94,7 +94,17 @@
       evalHost = { name, local, target }: nixpkgs.lib.nixosSystem {
         system = target;
         modules = [
-          (import ./hosts/instantiate.nix { localSystem = local; hostName = name; })
+          {
+            nixpkgs = (if (local != null) then {
+              buildPlatform = local;
+            } else {}) // {
+              # TODO: does the earlier `system` arg to nixosSystem make its way here?
+              hostPlatform.system = target;
+            };
+            # nixpkgs.buildPlatform = local;  # set by instantiate.nix instead
+            # nixpkgs.config.replaceStdenv = { pkgs }: pkgs.ccacheStdenv;
+          }
+          (import ./hosts/instantiate.nix { hostName = name; })
           self.nixosModules.default
           self.nixosModules.passthru
           {
@@ -103,12 +113,6 @@
               self.overlays.sane-all
             ];
           }
-          ({ lib, ... }: {
-            # TODO: does the earlier `system` arg to nixosSystem make its way here?
-            nixpkgs.hostPlatform.system = target;
-            # nixpkgs.buildPlatform = local;  # set by instantiate.nix instead
-            # nixpkgs.config.replaceStdenv = { pkgs }: pkgs.ccacheStdenv;
-          })
         ];
       };
     in {

hosts/instantiate.nix

@@ -1,10 +1,10 @@
 # trampoline from flake.nix into the specific host definition, while doing a tiny bit of common setup
 
 # args from flake-level `import`
-{ hostName, localSystem }:
+{ hostName }:
 
 # module args
-{ lib, ... }:
+{ ... }:
 
 {
   imports = [
@@ -14,5 +14,4 @@
   ];
 
   networking.hostName = hostName;
-  nixpkgs.buildPlatform = lib.mkIf (localSystem != null) localSystem;
 }

nixpatches/flake.nix

@@ -15,9 +15,15 @@
       patchedFlakeFor = system: import "${patchedPkgsFor system}/flake.nix";
       patchedFlakeOutputsFor = system:
         (patchedFlakeFor system).outputs { inherit self; };
+
+      extractBuildPlatform = nixosSystemArgs:
+        let
+          firstMod = builtins.head nixosSystemArgs.modules;
+        in
+          firstMod.nixpkgs.buildPlatform or nixosSystemArgs.system;
     in
     {
-      lib.nixosSystem = args: (patchedFlakeOutputsFor args.system).lib.nixosSystem args;
+      lib.nixosSystem = args: (patchedFlakeOutputsFor (extractBuildPlatform args)).lib.nixosSystem args;
 
       legacyPackages = builtins.mapAttrs
         (system: _: