fix(rubocop): pass --server for faster execution (#246)

This ensures a much quicker execution on 2nd and any subsequent run. Otherwise there's always a 1-3 seconds delay before formatting is being applied (as rubocop is not that quick).

From rubocop's documentation:

```
--server If a server process has not been started yet, start the server process and execute inspection with server.
```

lua/conform/formatters/rubocop.lua

@@ -6,6 +6,7 @@ return {
   },
   command = "rubocop",
   args = {
+    "--server",
     "-a",
     "-f",
     "quiet",