TST: Download data from G-Node GIN in notebook testing GHA workflow

Download data from `G-Node` `GIN` in notebook testing GHA workflow.

Fixes:
```
/home/runner/work/nifreeze/nifreeze/docs/notebooks/dmri_covariance.ipynb

hcpdata = np.load(DATA_PATH/"hcpdata.npz")

FileNotFoundError: [Errno 2] No such file or directory: '/home/runner/nifreeze-tests/hcpdata.npz'
```

raised for example in:
https://github.com/jhlegarreta/nifreeze/actions/runs/14814107025/job/41592479776#step:10:64

Author: jhlegarreta

.github/workflows/notebooks.yml

@@ -41,6 +41,11 @@ jobs:
         with:
           python-version: ${{ matrix.python-version }}
 
+      - name: Download data file from G-Node GIN
+        run: |
+          mkdir ${TEST_DATA_HOME}
+          curl -L -o "${TEST_DATA_HOME}/hcpdata.npz" https://gin.g-node.org/nipreps-data/tests-nifreeze/src/master/hcpdata.npz
+
       - name: Install TeX Live
         run: |
           sudo apt-get update