Replace the builder-fetch.sh script with better programming language

[laurentsimon]

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As part of https://github.com/slsa-framework/slsa-github-generator/pull/86, I used a script to download the builder's binary. We should use another programming language code instead, to avoid trusting the gh CLI binary and to make the code more readable. Maybe typscript is an option, and we can encapsulate it into a GH Action.

I think it's fine to have reliance on this binary for the first release, since it's an official GitHub project https://cli.github.com/

Please chime in if you think otherwise.