TST: Download SynthStrip model for skull stripping

Download `SynthStrip` model for skull stripping. Used by the fMRI BOLD
realignment notebook.

Fixes:
```
FileNotFoundError                         Traceback (most recent call last)
Cell In[4], line 19
     14 if not bmask_path.exists():
     15     bmsk_results = SynthStrip(
     16         in_file=str(avg_path),
     17         use_gpu=True,
     18     ).run(cwd=str(WORKDIR))
---> 19     copy(bmsk_results.outputs.out_mask,bmask_path)

(...)

FileNotFoundError: [Errno 2] No such file or directory:
 '/home/runner/tmp/nifreezedev/ismrm25/sub-01_task-mixedgamblestask_run-03_boldref_desc-brain_mask.nii.gz'
```

raised for example at:
https://github.com/jhlegarreta/nifreeze/actions/runs/14897027210/job/41841440531#step:11:102

Author: jhlegarreta

.github/workflows/notebooks.yml

@@ -47,6 +47,10 @@ jobs:
           pip install osfclient
           osf -p 39k4x fetch hcpdata.npz "${TEST_DATA_HOME}/hcpdata.npz"
 
+      - name: Download SynthStrip model
+        run: |
+          wget https://github.com/freesurfer/freesurfer/blob/dev/mri_synthstrip/synthstrip.1.pt -P ${FREESURFER_HOME}/models/
+
       - name: Install TeX Live
         run: |
           sudo apt-get update