fix(ci): use SSH on the merge bot (#607)

[claudioandre-br]

I'm seeing an access denied for the bot (but not for workflow_dispatch). Let's use an SSH user and see what happens.

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[claudioandre-br]

Ah, so it fails when the PR comes from a fork.

Okay, we can always revert this merge. PRs from forks will be processed via workflow_dispatch.

But let's test it before reverting.