Update provenance generator config

Author: abhisek

.github/workflows/goreleaser.yml

@@ -62,4 +62,6 @@ jobs:
     uses: slsa-framework/slsa-github-generator/.github/workflows/generator_generic_slsa3.yml@v1.4.0
     with:
       base64-subjects: "${{ needs.goreleaser.outputs.hashes }}"
-      upload-assets: true # upload to a new release
+      upload-assets: true
+      private-repository: true
+