Fix false critical on OMD backup job when check is delayed with more than 30 seconds

[tr3pan]

There is already a partial fix in #711

But issue still can happen if the CheckMK check runs delayed more 30 seconds. For example the scheduled backup is at 01:00:00, but the checkmk check for 00:59 happens at 00:59:46 for example. next_run < time.time() + 30 will be true since the next_run time is still from yesterday's backup.

I think 60 seconds is enough buffer, but adding additional 60 just to be on the safe side. The mkbackup command is scheduled by cron and if it is known the upper bound of when the backup changes its status, then the buffer time can be lowered.

[github-actions]

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[tr3pan]

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