Add GOPRIVATE to renovate-config.json

[mariduv]

This makes sure any go module in our org is fetched direct rather than through the public module proxy.

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Note

Low Risk
Renovate-only environment variable change with no runtime application impact.

Overview
Configures Renovate to set GOPRIVATE=github.com/trufflesecurity/* via customEnvVariables in renovate-config.json, so Go dependency updates resolve private org modules directly instead of through the public module proxy.

Also corrects the packageRules array closing (adds the trailing comma needed before the new top-level field).

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[mariduv]

I think this is only allowed for self-hosted. I actually don't see another way to set or hint goprivate for the renovate service.