fix: shorebird preview doesn't think I'm inside a project

[eseidel]

Running here:
https://github.com/shorebirdtech/samples/tree/main/flavors

PS C:\Users\micro\Documents\GitHub\samples\flavors> ..\..\shorebird\bin\shorebird preview
✓ Fetching apps (0.3s)
Which app would you like to preview?
❯ ◉  test_shorebird
  ◯  fresh_example
  ◯  fresh_dio
  ◯  openreads
  ◯  flutter_amazon_clone_bloc
  ◯  buck_the_critics
  ◯  test_3_16_4
  ◯  test_ios_mix
  ◯  flutter_and_friends
  ◯  fluttersaurus
  ◯  test_3_16_3
  ◯  flavors (stable)
  ◯  flavors (internal)
  ◯  flavors (stable)
  ◯  flavors (internal)
  ◯  test_3_13_9
  ◯  test_3_13_9
  ◯  mixed_test
  ◯  lima_test
  ◯  test_3_13_8
  ◯  flavors (stable)
  ◯  flavors (internal)
  ◯  test_3_13_4
  ◯  test_0_14_8
  ◯  test_0_14_7
  ◯  test_no_android
  ◯  shorebird_code_push_example
  ◯  test_3_13_1
  ◯  my_project
  ◯  test_counter
  ◯  time_shift
  ◯  shorebird_code_push
  ◯  fluttercon_demo
  ◯  ios_test
  ◯  test_app
  ◯  shorebird_hello
  ◯  hello_shorebird
  ◯  win_test

[eseidel]

We're jsut missing a "prompt for flavor" codepath:

 final shorebirdYaml = shorebirdEnv.getShorebirdYaml();
    final String? appId;
    if (results.wasParsed('app-id')) {
      appId = results['app-id'] as String;
    } else if (shorebirdYaml != null && shorebirdYaml.flavors == null) {
      appId = shorebirdYaml.appId;
    } else {
      appId = await promptForApp();
    }