root-signing: Enable github actions (#485)

This is needed for the internal "preprod" repository
and matches how root-signing-staging is setup.

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

github-sync/github-data/sigstore/repositories.yaml

@@ -1401,6 +1401,8 @@ repositories:
     hasIssues: true
     hasProjects: true
     hasWiki: true
+    pages:
+      buildType: workflow
     vulnerabilityAlerts: false
     visibility: public
     licenseTemplate: ""