Update deploy script to keep URL.

.github/workflows/deploy.yml

@@ -34,6 +34,7 @@ jobs:
           github_token: ${{ secrets.GITHUB_TOKEN }}
           # Build output to publish to the `gh-pages` branch:
           publish_dir: ./build
+          cname: rlgym.org
           # The following lines assign commit authorship to the official
           # GH-Actions bot for deploys to `gh-pages` branch:
           # https://github.com/actions/checkout/issues/13#issuecomment-724415212