Add RubyGems.org as environment.

Author: moroland

.github/workflows/gem-publish.yml

@@ -13,6 +13,8 @@ jobs:
       id-token: write # IMPORTANT: this permission is mandatory for trusted publishing
       contents: write # IMPORTANT: this permission is required for `rake release` to push the release tag
 
+    environment: RubyGems.org
+
     steps:
       # Set up
       - uses: actions/checkout@v4