AppX packaging

[Craftplacer]

The current msix package handling this is failing with following message:

Running "flutter pub get" in flutter_tools...                       4.1s
Error: No pubspec.yaml file found.
This command should be run from the root of your Flutter project.
Error: Process completed with exit code 1.

With following script:

env:
  SOURCE_PATH: '.\src\kaiteki'
working-directory: ${{ env.SOURCE_PATH }}
run: 'flutter pub run msix:create'