flutter: Update FLUTTER_ROOT in SDK symlink derivation

pkgs/development/compilers/flutter/sdk-symlink.nix

@@ -1,4 +1,6 @@
-{ symlinkJoin }: flutter:
+{ symlinkJoin
+, makeWrapper
+}: flutter:
 
 let
   self =
@@ -6,6 +8,12 @@ let
       name = "${flutter.name}-sdk-links";
       paths = [ flutter flutter.sdk ];
 
+      nativeBuildInputs = [ makeWrapper ];
+      postBuild = ''
+        wrapProgram "$out/bin/flutter" \
+          --set-default FLUTTER_ROOT "$out"
+      '';
+
       passthru = flutter.passthru // {
         # Update the SDK attribute.
         # This allows any modified SDK files to be included