chdir into /proc for determining default Config

There are users that run mkosi from / so let's use /proc instead.

Fixes #2786

mkosi/config.py

@@ -1556,7 +1556,7 @@ def default(cls) -> "Config":
 
  This prevents MkosiArgs being generated with defaults values implicitly.
  """
- with chdir("/"):
+ with chdir("/proc"):
  _, [config] = parse_config([])
 
  return config