[8.10](backport #3742) [CI] run MacOS 13 ARM unit tests only on main branch

[mergify]

This is an automatic backport of pull request #3742 done by Mergify.
Cherry-pick of 3155ea9 has failed:

On branch mergify/bp/8.10/pr-3742
Your branch is up to date with 'origin/8.10'.

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	both modified:   .buildkite/pipeline.yml

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[elasticmachine]

❕ Build Aborted

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• Start Time: 2023-11-10T21:21:53.118+0000

• Duration: 452 min 54 sec

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[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[mergify]

This pull request has not been merged yet. Could you please review and merge it @pazone? 🙏

[jlind23]

No more 8.10 hence closing cc @pierrehilbert