ci: Pin melos version to 2.9.0

Pin the melos version to 2.9.0 to the CI can pass

The current PRs fail with

Run melos bs
  melos bs
  shell: /usr/bin/bash -e {0}
  env:
    FLUTTER_ROOT: /opt/hostedtoolcache/flutter/beta-3.9.0-0.[2](https://github.com/gql-dart/ferry/actions/runs/4649666127/jobs/8228167917?pr=499#step:5:2).pre-x64
    PUB_CACHE: /opt/hostedtoolcache/flutter/beta-[3](https://github.com/gql-dart/ferry/actions/runs/4649666127/jobs/8228167917?pr=499#step:5:3).9.0-0.2.pre-x6[4](https://github.com/gql-dart/ferry/actions/runs/4649666127/jobs/8228167917?pr=499#step:5:4)/.pub-cache
Found a melos.yaml file in "/home/runner/work/ferry/ferry" but no local installation of Melos.

From version 3.0.0, the melos package must be installed in a pubspec.yaml file next to the melos.yaml file.

For more information on migrating to version 3.0.0, see: https://melos.invertase.dev/guides/migrations#200-to-300

To migrate at a later time, ensure you have version 2.[9](https://github.com/gql-dart/ferry/actions/runs/4649666127/jobs/8228167917?pr=499#step:5:10).0 or below installed: dart pub global activate melos 2.9.0
Error: Process completed with exit code 1.

(Like https://github.com/gql-dart/ferry/pull/499)

gql-dart / ferry

ci: Pin melos version to 2.9.0 #506