sync/shared-config: Mark governance-private as a custom RuboCop repo

- There's a single script in here that's used once a year, we don't need all of the RuboCop rules and it was causing a lot of weirdness with the Sorbet cops.

.github/actions/sync/shared-config.rb

@@ -78,6 +78,7 @@ def git(*args)
   mass-bottling-tracker-private
   orka_api_client
   ruby-macho
+  governance-private
 ].freeze
 custom_dependabot_repos = %w[
   .github