👮 Support adding a specific clone token

[MadsBogeskov]

Overall Idea

Some shuttle repositories might not be accessible from the runner, e.g. if all repositories are placed in a private GitHub organisation. Historically this has been solved by overriding the clone path in the GitHub Action like so:

- name: Test
  run: shuttle run build_framework github_token=${{ secrets.GITHUB_TOKEN }} --plan https://${{ secrets.IOS_BOT_TOKEN }}@github.com/lunarway/shuttle-ios-plan.git#main

Instead we would like to just being able to use the plan specified in the shuttle file and just set a access token from inside the action instead, making it look like this:

- name: Build
  run: shuttle run build_framework
  env:
    SHUTTLE_GIT_TOKEN: ${{ steps.token.outputs.token }}