[ci] add more permissions for create-pull-request

[chunhtai]

It seems like create pull request also needs content write permission

https://github.com/marketplace/actions/create-pull-request

current workflow error

  /usr/bin/git push --force-with-lease origin go_router-23310566202-1:refs/heads/go_router-23310566202-1
  remote: Permission to flutter/packages.git denied to github-actions[bot].
  fatal: unable to access 'https://github.com/flutter/packages/': The requested URL returned error: 403
  Error: The process '/usr/bin/git' failed with exit code 128

https://github.com/flutter/packages/actions/runs/23310566202/job/67796535425#step:3:884

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