Can't create user space when using the Joint Access Storage Driver as non root

## Describe the bug
When deploying OCIS with the posix storage driver using systemd and user ocis (as per the docs) the service fails to create the user space.

## Steps to reproduce
Here's my full deployment procedure runable from a bash script:

```sh
wget -O /usr/local/bin/ocis https://github.com/owncloud/ocis/releases/download/v6.2.0/ocis-6.2.0-linux-amd64
chmod +x /usr/local/bin/ocis

useradd --system --no-create-home --shell=/sbin/nologin ocis

mkdir -p /var/lib/ocis/posix-storage
mkdir -p /etc/ocis
touch /etc/ocis/ocis.env
chown -R ocis:ocis /var/lib/ocis
chown -R ocis:ocis /etc/ocis

cat > /etc/ocis/ocis.env << EOF
OCIS_URL=https://owncloud.homelab:9200
PROXY_HTTP_ADDR=0.0.0.0:9200

OCIS_LOG_LEVEL=warn

OCIS_CONFIG_DIR=/etc/ocis
OCIS_BASE_DATA_PATH=/var/lib/ocis

STORAGE_USERS_DRIVER="posix"
STORAGE_USERS_POSIX_ROOT="/var/lib/ocis/posix-storage"
STORAGE_USERS_POSIX_WATCH_TYPE="inotifywait"
STORAGE_USERS_ID_CACHE_STORE="nats-js-kv"
STORAGE_USERS_ID_CACHE_STORE_NODES="localhost:9233"
EOF

sudo -u ocis ocis init --config-path /etc/ocis

cat > /etc/systemd/system/ocis.service << EOF
[Unit]
Description=OCIS server

[Service]
Type=simple
User=ocis
Group=ocis
EnvironmentFile=/etc/ocis/ocis.env
ExecStart=/usr/local/bin/ocis server
Restart=always

[Install]
WantedBy=multi-user.target
EOF

systemctl daemon-reload
systemctl enable --now ocis
```

## Expected behavior
The user space is created and the service run as usual

## Actual behavior
The user space is not created and the following errors occurs:

```console
Aug 05 14:14:28 owncloud ocis[5562]: {"level":"error","service":"storage-users","pkg":"rgrpc","traceid":"a03ca61801683eeb9d986de767a85144","error":"open .flock: permission denied","status":{"code":15,"message":"error creating space","trace":"a03ca61801683eeb9d986de767a85144"},"request":{"opaque":{"map":{"space_id":{"decoder":"plain","value":"M2ZmYzIyZGMtZjE5ZS00ZWU2LWJjYTMtODY0NDA1N2YyNzlk"}}},"owner":{"id":{"idp":"https://owncloud.homelab:9200","opaque_id":"3ffc22dc-f19e-4ee6-bca3-8644057f279d","type":1},"username":"admin","mail":"admin@example.org","display_name":"Admin","uid_number":99,"gid_number":99},"type":"personal","name":"Admin"},"time":"2024-08-05T14:14:28Z","message":"failed to create storage space"}
Aug 05 14:14:28 owncloud ocis[5562]: {"level":"error","service":"proxy","error":"gateway: grpc failed with code CODE_INTERNAL","time":"2024-08-05T14:14:28Z","message":"error when calling Createhome"}
```

## Additional context
- If I run the service as the root user instead everything runs correctly.
- I have the same errors if I create a home for user ocis and set the storage in the home directory.
- I have the same errors if I run `chmod -R 777 /var/lib/ocis/posix-storage`
- Removing the posix storage driver section makes the service run correctly.

Thanks!