ep: Only fetch experiments if cx.is_staff (#52739)

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

Closes #ISSUE

Release Notes:

- N/A

crates/edit_prediction/src/edit_prediction.rs

@@ -778,8 +778,11 @@ impl EditPredictionStore {
             while current_user.borrow().is_none() {
                 current_user.next().await;
             }
+
             this.update(cx, |this, cx| {
-                this.refresh_available_experiments(cx);
+                if cx.is_staff() {
+                    this.refresh_available_experiments(cx);
+                }
             })
             .log_err();
         });