iOS icon generation fails with custom xcodeproj path when using flavors

## Description
When using flavor-specific launcher icons with a custom `xcodeproj_path`, the tool fails to update the project file despite following the documented flavor configuration pattern (`flutter_launcher_icons-.yaml`).

## Steps to Reproduce
1. Create a Flutter project with flavors (dev, hml, prod) and custom iOS project name
2. Create flavor-specific icon configuration files following the documentation pattern:

Example `flutter_launcher_icons-hml.yaml`:
```yaml
flutter_launcher_icons:
android: true
ios: true
image_path: "assets/icons/ic_launcher_hml.png"
remove_alpha_ios: true
xcodeproj_path: "ios/MyApp.xcodeproj"
```

3. Run the flavor-specific command:
```bash
flutter pub get
dart run flutter_launcher_icons -f flutter_launcher_icons-hml.yaml
```

## Current Behavior
The tool:
1. Successfully generates the flavor-specific icons in Assets.xcassets
2. Fails with path error, still looking for default Runner.xcodeproj:

```
✓ Successfully generated launcher icons
Unhandled exception:
PathNotFoundException: Cannot open file, path = 'ios/Runner.xcodeproj/project.pbxproj' (OS Error: No such file or directory, errno = 2)
```

## Expected Behavior
The tool should:
1. Generate the flavor-specific icons (currently working)
2. Use the specified `xcodeproj_path` for project file updates
3. Successfully complete the flavor-specific icon update process

## Environment
- Flutter version: 3.19.0
- flutter_launcher_icons version: 0.14.3
- Operating System: macOS Sonoma 14.3.1

## Additional Context
- The documentation explicitly supports flavors through `flutter_launcher_icons-.yaml` files
- Icon generation works correctly for flavors
- The issue appears specific to project file updates when using both flavors and custom xcodeproj paths
- Project structure follows Flutter flavor best practices with separate configurations for dev, hml, and prod environments

## Related Documentation
From the official documentation:
> Create a Flutter Launcher Icons configuration file for your flavor. The config file is called flutter_launcher_icons-.yaml by replacing by the name of your desired flavor.

Start with the flavor entry point `dart run flutter_launcher_icons -f flutter_launcher_icons-hml.yaml` and the documented `flutter_launcher_icons-.yaml` configuration, focusing on how `xcodeproj_path: "ios/MyApp.xcodeproj"` is used during project updates. Reproduce the failure and inspect the attempted `ios/Runner.xcodeproj/project.pbxproj` path; done means the custom project file is updated and the command completes successfully.

Written by the indexing model from the issue text.