Investigate the use of stdin indirection to reduce chance of print by builder

[laurentsimon]

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[laurentsimon]

This was done in one of the previous PRs https://github.com/asraa/slsa-on-github/blob/main/.github/workflows/slsa-builder-go.yml#L256

What I don't like is that stderr is also sent to /dev/null, which will make it hard to troubleshoot. My plan is to change that to only redirect stdout to /dev/null.

I'll verify whether stderr can use ::set-output command, I expect not..