`avoid_unnecessary_return_variable` incorrectly triggers for variables used in multiple return paths


```dart
String test() {
  final someCondition = 1 == 1;
  final a = 'test';
  if (someCondition) return a;

  return 'something $a';
}
```

**Expected behavior:**
No lint - the variable `a` is used in multiple return statements, so it is not unnecessary.

**Actual behavior:**
The lint triggers on `return a;` incorrectly suggesting that the variable is only used in a single return.