[ffigen] Add variable substitutions for ObjC SDKs

[liamappelbe]

• $XCODE: xcode-select -p
  • eg /Applications/Xcode.app/Contents/Developer
• $IOS_SDK: xcrun --show-sdk-path --sdk iphoneos
  • eg /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS18.2.sdk
• $MACOS_SDK: xcrun --show-sdk-path --sdk macosx
  • eg /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.2.sdk

Fixes #471

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?

Changelog Entry ✔️

Package	Changed Files

Changes to files need to be accounted for in their respective changelogs.

API leaks ✔️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbols

License Headers ✔️

// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file // for details. All rights reserved. Use of this source code is governed by a // BSD-style license that can be found in the LICENSE file. 

Files
no missing headers

All source files should start with a license header.

Unrelated files missing license headers

Files
pkgs/jni/lib/src/third_party/generated_bindings.dart
pkgs/objective_c/lib/src/ns_input_stream.dart

[coveralls]

coverage: 90.942% (+0.5%) from 90.423%
when pulling 368b3ca on xcodevar
into 6ddc868 on main.