Follow up to #54206

`on_flags_ready` relied on the fact that the `FeatureFlagStore` was only
set once the flags had been received from the server.
However, after #54206 the global gets instantiated earlier, without the
flags being resolved.

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A