Stop building both branch & PR with pushes

Build pushes to the main branch and open PRs, but not pushes to other
branches. Allow workflow_dispatch to build them manually if we want to.

.github/workflows/test.yml

@@ -1,5 +1,10 @@
 name: Test
-on: [push, pull_request]
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+  workflow_dispatch:
 jobs:
   test:
     strategy: