[WIP] Fix permission check on $SECURITY_AGENT_CONFIG_FILE

[sietse]

$SECURITY_AGENT_CONFIG_FILE does not need to be executable, merely readable.

This fixes spurious error /etc/datadog-agent/security-agent.yaml not found. Exiting datadog-agent-security, caused by the startupscript wrongly checking for an executable config file instead of a readable one.

TODO:

• check other (non-Debian) service files for faulty check logic
• go down PR checklist / read Contributing.md

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• If applicable, the config template has been updated.

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