fix(ci): preserve STT e2e provider JSON

[ComputelessComputer]

Forward detected provider output with printf so workflow matrices receive valid JSON.

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Note

Low Risk
Low risk CI-only change that adjusts how a job output is forwarded; main risk is misquoting the output and breaking the E2E matrix expansion.

Overview
Fixes the STT E2E GitHub Actions workflow to preserve the detected providers JSON when forwarding it from detect-changes into the setup job output.

Replaces echo with printf (and safer quoting) so fromJson(needs.setup.outputs.providers) reliably receives valid JSON for the provider matrix.

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