cleanup: don’t cleanup periodic clean file.

Cleaning up this file means that cleanup will run again prematurely.

Library/Homebrew/cleanup.rb

@@ -300,6 +300,8 @@ module Homebrew
       entries ||= [cache, cache/"Cask"].select(&:directory?).flat_map(&:children)
 
       entries.each do |path|
+        next if path == PERIODIC_CLEAN_FILE
+
         FileUtils.chmod_R 0755, path if path.go_cache_directory? && !dry_run?
         next cleanup_path(path) { path.unlink } if path.incomplete?
         next cleanup_path(path) { FileUtils.rm_rf path } if path.nested_cache?