Fix checks using `waitmax` and `run_cached` in Linux agent

[sparunakian]

General information

Since checkmk 2.2, some checks (like chrony) are not returning any data and are presumed missing/vanished. This is due to a regression.

Bug reports

checkmk 2.2.0p7 installed on Debian 12 checkmk Linux agent 2.2.0p7, over SSH, on Debian 12 (but other Linux distributions are affected.) chrony on the monitored servers

There is no step to reproduce the bug, it appears immediately.

The use of both run_cached and waitmax in some agent sections is causing this. On servers with the timeout binary, waitmax is defined as a function calling timeout In the run_cached function a subshell is started and the waitmax function is not known in this context. If there is no waitmax binary on the system, the check silently fails.

In the 2.1 version of the agent, there was export -f waitmax just after the definition of the waitmax function, allowing it to be callable in the subshell started in the run_cached function.

Proposed changes

This MR reintroduces the missing export.

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