Fix extensions directory search
While deploying https://github.com/huggingface/datasets-server/pull/2482 in staging, this error appeared:
```
INFO: 2024-02-23 17:03:36,387 - root - /search dataset='asoria/bolivian-recipes' config='default' split='same' query='beef' offset=0 length=100
DEBUG: 2024-02-23 17:03:36,413 - root - connect to index file /storage/duckdb-index/downloads/0.10.0/asoria-bolivian-recipes-5bb26f91/default/same/index.duckdb
ERROR: 2024-02-23 17:03:36,414 - root - Unexpected error.
Traceback (most recent call last):
  File "/src/services/search/src/search/routes/search.py", line 202, in search_endpoint
    num_rows_total, pa_table = await anyio.to_thread.run_sync(
  File "/src/services/search/.venv/lib/python3.9/site-packages/anyio/to_thread.py", line 33, in run_sync
    return await get_asynclib().run_sync_in_worker_thread(
  File "/src/services/search/.venv/lib/python3.9/site-packages/anyio/_backends/_asyncio.py", line 877, in run_sync_in_worker_thread
    return await future
  File "/src/services/search/.venv/lib/python3.9/site-packages/anyio/_backends/_asyncio.py", line 807, in run
    result = context.run(func, *args)
  File "/src/services/search/src/search/routes/search.py", line 64, in full_text_search
    with duckdb_connect(database=index_file_location) as con:
  File "/src/services/search/src/search/duckdb_connection.py", line 10, in duckdb_connect
    con.sql(LOAD_FTS_SAFE_COMMAND)
duckdb.duckdb.IOException: IO Error: Failed to create directory "/.duckdb/"!
```
Will set the extensions directory to see if it fixes the issue same as in workers https://github.com/huggingface/datasets-server/pull/1439.
Fix extensions directory search: While deploying https://github.com/huggingface/datasets-server/pull/2482 in staging, this error appeared:
```
INFO: 2024-02-23 17:03:36,387 - root - /search dataset='asoria/bolivian-recipes' config='default' split='same' query='beef' offset=0 length=100
DEBUG: 2024-02-23 17:03:36,413 - root - connect to index file /storage/duckdb-index/downloads/0.10.0/asoria-bolivian-recipes-5bb26f91/default/same/index.duckdb
ERROR: 2024-02-23 17:03:36,414 - root - Unexpected error.
Traceback (most recent call last):
  File "/src/services/search/src/search/routes/search.py", line 202, in search_endpoint
    num_rows_total, pa_table = await anyio.to_thread.run_sync(
  File "/src/services/search/.venv/lib/python3.9/site-packages/anyio/to_thread.py", line 33, in run_sync
    return await get_asynclib().run_sync_in_worker_thread(
  File "/src/services/search/.venv/lib/python3.9/site-packages/anyio/_backends/_asyncio.py", line 877, in run_sync_in_worker_thread
    return await future
  File "/src/services/search/.venv/lib/python3.9/site-packages/anyio/_backends/_asyncio.py", line 807, in run
    result = context.run(func, *args)
  File "/src/services/search/src/search/routes/search.py", line 64, in full_text_search
    with duckdb_connect(database=index_file_location) as con:
  File "/src/services/search/src/search/duckdb_connection.py", line 10, in duckdb_connect
    con.sql(LOAD_FTS_SAFE_COMMAND)
duckdb.duckdb.IOException: IO Error: Failed to create directory "/.duckdb/"!
```
Will set the extensions directory to see if it fixes the issue same as in workers https://github.com/huggingface/datasets-server/pull/1439.