Race condition between `update_barcode_and_snapshots` and oav snapshot last saved path

`test_load_centre_collect_full_plan` has a line
```
robot_load_cb.expeye.update_barcode_and_snapshots.assert_called_once_with(
1234,
"BARCODE",
"/tmp/dls/i03/data/2024/cm31105-4/auto/123457/xraycentring/snapshots/160705_webcam_after_load.png",
"/tmp/snapshot1.png",
)
```
(currently commented out)
which fails with `after_snapshot` being empty due to the callback being invoked before `MJPG._save_image` updates the `last_saved_path` when doing the post-processing for the OAV snapshot

## Acceptance Criteria
- Test passes