Don't run deploy.yml twice for every PR

This restricts CI runs on push to just the main branch, so that PRs
don't run the same workflow twice on every push.

.github/workflows/deploy.yml

@@ -3,6 +3,8 @@ name: Build, Test, and (maybe) Deploy React Application
 on:
   # Triggers the workflow on push or pull request events but only for the main branch
   push:
+    branches:
+      - main
   pull_request:
   workflow_dispatch: