build: T6776: use the official Zabbix repo for zabbix-agent2

[dmbaturin]

Change Summary

Use the official repo for installing zabbix-agent2 to keep it up to date.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

Component(s) name

Build configuration.

Proposed changes

Debian maintainers, oddly enough, are not updating the zabbix-agent2 6.x package in Bookworm, despite known vulnerabilities. There's zabbix-agent2 7.x in bookworm-backports, but 6.x is the current LTS version, and 7.x may not work with older Zabbix servers, so we cannot "just" upgrade to 7.x.

Using the official repo solves that problem, since they include the latest versions there. One peculiarity is that they have separate repos for x86-64 and aarch64, which is why URLs in files for different architectures are different.

How to test

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

[github-actions]

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