[bug]: upload symbols fails with Flutter 3.44 because the SourcePackages have moved

### Is there an existing issue for this?

- [x] I have searched the existing issues.

### CLI Version

1.3.2

### Firebase Tools version

15.18.0

### Flutter Doctor Output

[✓] Flutter (Channel stable, 3.44.0, on macOS 26.5 25F71 darwin-arm64, locale de-DE) [301ms]
• Flutter version 3.44.0 on channel stable at /Users/xxx/sdk/flutter
• Upstream repository https://github.com/flutter/flutter.git
• Framework revision 559ffa3f75 (6 days ago), 2026-05-15 14:13:13 -0700
• Engine revision 4c525dac5e
• Dart version 3.12.0
• DevTools version 2.57.0
• Feature flags: enable-web, enable-linux-desktop, enable-macos-desktop, enable-windows-desktop, enable-android, enable-ios, cli-animations, enable-native-assets,
enable-swift-package-manager, omit-legacy-version-file, enable-lldb-debugging, enable-uiscene-migration

[✓] Android toolchain - develop for Android devices (Android SDK version 36.1.0) [1.360ms]
• Android SDK at /Users/xxx/sdk/android
• Emulator version 36.5.11.0 (build_id 15261927) (CL:N/A)
• Platform android-36.1, build-tools 36.1.0
• ANDROID_HOME = /Users/xxx/sdk/android
• ANDROID_SDK_ROOT = /Users/xxx/sdk/android
• Java binary at: /Applications/Android Studio.app/Contents/jbr/Contents/Home/bin/java
This is the JDK bundled with the latest Android Studio installation on this machine.
To manually set the JDK path, use: `flutter config --jdk-dir="path/to/jdk"`.
• Java version OpenJDK Runtime Environment (build 21.0.10+-117844308-b1163.108)
• All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 26.4.1) [1.508ms]
• Xcode at /Applications/Xcode.app/Contents/Developer
• Build 17E202
• CocoaPods version 1.16.2

[✓] Chrome - develop for the web [4ms]
• Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Network resources [185ms]
• All expected network resources are available.

• No issues found!

### Description

The build fails because with flutter 3.44, the `SourcePackages` are no longer at `DERIVED_DATA_PATH`, but at `BUILD_ROOT`.

Trying to build a clean project produces the following error:

```
Unhandled exception:
ProcessException: No such file or directory
Command:
/Users/XXX/Library/Developer/Xcode/DerivedData/Runner-asnmalsvsfbtamfmomnltzgnmkzh/SourcePackages/checkouts/firebase-ios-sdk/Crashlytics/run --validate --flutter-project .../app_id_file.json
```

Checking the derived data directory reveals there is no `SourcePackages` directory:

However, checking the `build/ios` dir reveales it has moved there:

### Steps to reproduce

1. Create a new flutter project with firebase_crashlytics or run `flutter clean` on an existing one (as otherwise the `SourcePackages` might still exist from previous builds)
2. Build the project.

### Expected behavior

It uploads the symbols correctly.

### Screenshots

See Description

### Additional context and comments

_No response_