[ci] Comment runtime changes on PRs

[driazati]

trafficstars

This implements #12781 but needs to wait for #12695 before it can merge

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• cc @areusch, @gigiblender, @Mousius _{See #10317 for details}
• Unable to run tests bot because main failed to pass CI at bb80f19ea8493af71c6130301f1b479143d213ee.
• Built docs for commit a379ebb0fd14e91fd8f7194f2f6552e0445242a4 can be found here.

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