Only run publish workflow on upstream repo

When syncing forks, they also try to publish (which doesn't make a lot of sense and doesn't work)

Ref: rubocop/rubocop-rspec#2050

Author: Earlopain

.github/workflows/publish.yml

@@ -7,6 +7,7 @@ jobs:
   publish:
     name: Publish to RubyGems
     runs-on: ubuntu-latest
+    if: github.repository_owner == 'rubocop'
     permissions:
       contents: write
       id-token: write