Avoid using special characters for cache keys, which was leading to a warning message on support bundle generation.
Use a separate file for the credentials cache, so it does not trigger
SaveableListeners which in turns could cause slow credentials lookup (i.e. Job Config History plugin tracking all saves on the cache file).
Note that this change will clear the potentially existing cache file, so it requires a cache re-build (it is gradually re-built as credentials are accessed, nothing to do from a user perspective).