ignore .ready and .backup files in find query

[Aeon]

• .ready and .backup files may be present in the WAL directory, but are not valid arguments to pg_archivecleanup
• using cut is probably cheaper than awk

[Napsty]

Handled in https://github.com/Napsty/scripts/commit/d5229457173cdd18e66569cd2c604326ccf4b22b Keeping awk for now.