flutter: Move artifact override logic to fetch-artifacts.nix

pkgs/development/compilers/flutter/artifacts/fetch-artifacts.nix

@@ -22,7 +22,14 @@ let
 in
 runCommand "flutter-artifacts-${platform}"
 {
-  nativeBuildInputs = [ flutter unzip ];
+  nativeBuildInputs = [
+    (flutter.override {
+      # Use a version of Flutter with just enough capabilities to download
+      # artifacts.
+      supportedTargetPlatforms = [ ];
+    })
+    unzip
+  ];
 
   NIX_FLUTTER_TOOLS_VM_OPTIONS = "--root-certs-file=${cacert}/etc/ssl/certs/ca-bundle.crt";
 

pkgs/development/compilers/flutter/artifacts/prepare-artifacts.nix

@@ -1,10 +1,11 @@
-{ stdenv
-, lib
+{ lib
+, stdenv
+, callPackage
 , autoPatchelfHook
 , src
 }:
 
-stdenv.mkDerivation {
+(stdenv.mkDerivation {
   inherit (src) name;
   inherit src;
 
@@ -18,4 +19,8 @@ stdenv.mkDerivation {
 
     runHook postInstall
   '';
-}
+}).overrideAttrs (
+  if builtins.pathExists ./overrides/${src.platform}.nix
+  then callPackage ./overrides/${src.platform}.nix { }
+  else ({ ... }: { })
+)

pkgs/development/compilers/flutter/wrapper.nix

@@ -50,19 +50,10 @@ let
     (callPackage ./artifacts/prepare-artifacts.nix {
       src = callPackage ./artifacts/fetch-artifacts.nix {
         inherit platform;
-        # Use a version of Flutter with just enough capabilities to download
-        # artifacts.
-        flutter = callPackage ./wrapper.nix {
-          inherit flutter;
-          supportedTargetPlatforms = [ ];
-        };
+        flutter = callPackage ./wrapper.nix { inherit flutter; };
         hash = artifactHashes.${platform}.${stdenv.hostPlatform.system} or "";
       };
-    }).overrideAttrs (
-      if builtins.pathExists ./artifacts/overrides/${platform}.nix
-      then callPackage ./artifacts/overrides/${platform}.nix { }
-      else ({ ... }: { })
-    ));
+    }));
 
   cacheDir = symlinkJoin rec {
     name = "flutter-cache-dir";