ENH: Store bold realignment results in reachable path

Store bold realignment results in a folder that actually exists.

Fixes:
```
 FAILED docs/notebooks/bold_realignment.ipynb::bold_realignment.ipynb
 - PermissionError: [Errno 13] Permission denied: '/data/derivatives'
```

raised for example in:
https://github.com/nipreps/nifreeze/actions/runs/12847776534/job/35824459079?pr=35#step:12:1684

docs/notebooks/bold_realignment.ipynb

@@ -34,7 +34,7 @@
     "WORKDIR = Path.home() / \"tmp\" / \"nifreezedev\" / \"ismrm25\"\n",
     "WORKDIR.mkdir(parents=True, exist_ok=True)\n",
     "\n",
-    "OUTPUT_DIR = Path(\"/data/derivatives\") / \"nifreeze-ismrm25-exp2\"\n",
+    "OUTPUT_DIR = Path.home() / \"tmp\" / \"nifreezedev\" / \"ismrm25\" / \"nifreeze-ismrm25-exp2\"\n",
     "OUTPUT_DIR.mkdir(exist_ok=True, parents=True)"
    ]
   },