fix borg config --cache (minimal change)

this was broken by previous changeset.

now doing just a minimal change for better diff display,
see also next commit.

src/borg/archiver.py

@@ -1769,6 +1769,11 @@ class Archiver:
             section = args.cache and "cache" or "repository"
             name = args.name
 
+        if args.cache:
+            manifest, key = Manifest.load(repository, (Manifest.Operation.WRITE,))
+            assert_secure(repository, manifest)
+            cache = Cache(repository, key, manifest, lock_wait=self.lock_wait)
+
         if args.cache:
             cache.cache_config.load()
             config = cache.cache_config._config
@@ -1797,6 +1802,9 @@ class Archiver:
             except (configparser.NoOptionError, configparser.NoSectionError) as e:
                 print(e, file=sys.stderr)
                 return EXIT_WARNING
+
+        if args.cache:
+            cache.close()
         return EXIT_SUCCESS
 
     def do_debug_info(self, args):