Fix stable flutter version

[donpui]

Add fixed stable version for flutter in CI to avoid new version surprises.

Currently with flutter version 3.7.0 fails on CI: https://github.com/LeastAuthority/destiny/actions/runs/4003941775 and we have issue for that: https://github.com/LeastAuthority/destiny/issues/226

---

Code Review Checklist

• [ ] Description accurately reflects what changes are being made.
• [ ] Description explains why the changes are being made (or references an issue containing one).
• [ ] The PR appropriately sized.
• [ ] New code has enough tests.
• [ ] New code has enough documentation to answer "how do I use it?" and "what does it do?".
• [ ] Existing documentation is up-to-date, if impacted.