Fix the argument name in open_cache

This fixes a bug introduced in aeb4dffe:

Parsing /etc/obs-proxy/proxy.conf
Traceback (most recent call last):
  File "/usr/local/bin/obs-proxy-server", line 33, in <module>
    sys.exit(load_entry_point('obs-proxy==0', 'console_scripts', 'obs-proxy-server')())
  File "/usr/local/lib/python3.9/dist-packages/obs_proxy-0-py3.9.egg/obs_proxy/server.py", line 549, in run                                                                                  
  File "/usr/local/lib/python3.9/dist-packages/obs_proxy-0-py3.9.egg/obs_proxy/utils.py", line 212, in open_cache                                                                            
UnboundLocalError: local variable 'cache_file' referenced before assignment