chore: fix publish ci (take 2)

[josxha]

Fixes in this pull request:

• the publish-single.yml workflow made no usage of the working-directory before
• use setup-dart because setup-flutter doesn't support automatic publishing 😥
• Use --skip-validation so that the missing flutter sdk causes no problem
• Remove pub get since without a validation we don't need to fetch dependencies
• and last but not least the trickiest one: Remove the packages from the .pubignore file because pub isn't able to see its contents otherwise
  • error message with the previous .pubignore: https://github.com/josxha/flutter-maplibre-gl/actions/runs/9181958689/job/25249862399
  • output after the packages are no longer ignored: https://github.com/josxha/flutter-maplibre-gl/actions/runs/9181975713/job/25249917620