nixos/nginx: fixup permissions for Nginx state dir

The commit b0bbacb5 was a bit too fast
It did set executable bit for log files.

Also, it didn't account for other directories in state dir:
```
 # ls -la /var/spool/nginx/
total 32
drwxr-x--- 8 nginx nginx 4096 Dec 26 12:00 .
drwxr-xr-x 4 root  root  4096 Oct 10 20:24 ..
drwx------ 2 root  root  4096 Oct 10 20:24 client_body_temp
drwx------ 2 root  root  4096 Oct 10 20:24 fastcgi_temp
drwxr-x--- 2 nginx nginx 4096 Dec 26 12:00 logs
drwx------ 2 root  root  4096 Oct 10 20:24 proxy_temp
drwx------ 2 root  root  4096 Oct 10 20:24 scgi_temp
drwx------ 2 root  root  4096 Oct 10 20:24 uwsgi_temp
```

With proposed change, only ownership is changed for state files, and mode is left as is
except that statedir/logs is now group accessible.