Werk #5512: cmk-update-agent: Make executable-Format-rule relevant for Linux only

After introducing the binary Format for the agent updater, we added a rule to the agent updater ruleset that enables the user to choose the format of the cmk-update-agent executable.

Hence the packaged binary format is only runnable on Linux, this rule now gets ignored for all other OSes.

Please note: If you want to enable automatic updates on Linux hosts that run on an architecture other than x86-64 or x86, you have to explicitibly choose to deploy the python script format, since the binary formats are not runnable on these architectures.

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