[BUG] launching with flavor (-f) option generates icons for ALL flavors instead of the desired only

### :information_source: Info

Version: `v0.8.0`

### :speech_balloon: Description

I have set up 3 flavors in my app: flavor1, flavor2 and flavor3.
For flavor1 and flavor2 I already had generated the launcher icons beforehand.

Now I only want to generate icons for flavor3 only. I created an own yaml file for flavor3 called `flutter_launcher_icons-flavor3.yaml`. I execute the following command in the terminal:
```shell
flutter pub run flutter_launcher_icons:main -f flutter_launcher_icons-flavor3.yaml
```
and get the output
```
Flavor: flavor3
• Building iOS launcher icon for flavor3

Flavor: flavor2
• Building iOS launcher icon for flavor2

Flavor: flavor1
• Building iOS launcher icon for flavor1

✓ Successfully generated launcher icons for flavors
```
The previously generated icons of flavor1 and flavor2 are overwritten. However, I would only expect that icons for flavor3 are generated and the other two are left untouched...

### :scroll: Pubspec.yaml

My `flutter_launcher_icons-flavor3.yaml` file for flavor3:
```yaml
flutter_icons:
android: false
ios: true
image_path: "assets/logos/flavor3.png"
```

Start by running `flutter pub run flutter_launcher_icons:main -f flutter_launcher_icons-flavor3.yaml` with the provided flavor configuration and inspect the CLI entry point that selects flavors. Confirm that only flavor3 is processed and that existing flavor1 and flavor2 icons remain unchanged.

Written by the indexing model from the issue text.