CI: Block "merge queue" if squash merge label is set

Squash merge should be performed via admins, adding to the queue would be a mistake and should be blocked

Feasible with GitHub actions, but we need to be sure there's a good developer experience (don't show it as falling to users)

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I should have left this to be squashed. My bad

_Originally posted by @BrayanDSO in https://github.com/ankidroid/Anki-Android/pull/14904#issuecomment-1847954546_