Don't try to run the release-prod step from non-main branches

The whole workflow is normally only triggered on pushes to main, but
when testing the workflow, we might want to trigger it from another
branch.

.github/workflows/release.yml

@@ -62,6 +62,7 @@ jobs:
       run: scripts/deploy-to-s3.js --bucket ${{ env.S3_BUCKET }} --tag qa --no-cache-entry
 
   release-prod:
+    if: github.ref_name == 'main'
     needs: release-qa
     runs-on: ubuntu-latest
     environment: production