Extract truffleruby workflow

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Background: #1032

I understand that constantly testing IRB with TruffleRuby-head is important to help the project cache issues early, but the failed build leads to a lot of noise in the CI workflow.

So in this PR, I hope to achieve 2 things by having a dedicated build configuration for TruffleRuby head:

1. We can very simply disable the CI runs through GH Actions' UI when there's a known issue, and re-enable it easily later, all without changing code.
2. Since TruffleRuby doesn't run most of the steps in the original build configuration, extracting it out makes the steps it DOES run more obvious.

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cc @andrykonchin @eregon