Werk #6443: Fixed possible "omd backup" error when files vanish during backup

When Check_MK creates temporary files while writing configuration files or similar, a parallel running "omd backup" job may try to backup those temporary files. When these backup files are finished and moved to the destination path, the backup may terminate with an error like this:

OSError: [Errno 2] No such file or directory: '/omd/sites/[SITE]/var/check_mk/web/[USER]/.last_pw_change.mk.newPm8BK8'

This issue is now prevented by skipping those temporary files during backup.

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