Replace hardcoded iOS build dir with config-driven lookup (#5732)

Related: b5f763b45e454863eb5baa10132e5e2df59d6987

packages/flutter_tools/lib/src/ios/mac.dart

diff --git a/packages/flutter_tools/lib/src/ios/mac.dart b/packages/flutter_tools/lib/src/ios/mac.dart
index e6e8ebe..df2be09 100644
--- a/packages/flutter_tools/lib/src/ios/mac.dart
+++ b/packages/flutter_tools/lib/src/ios/mac.dart
@@ -131,7 +131,7 @@
       commands.addAll(<String>[
         '-workspace', path.basename(entity.path),
         '-scheme', path.basenameWithoutExtension(entity.path),
-        "BUILD_DIR=${path.absolute(app.appDirectory, 'build')}",
+        "BUILD_DIR=${path.absolute(getIosBuildDirectory())}",
       ]);
       break;
     }