GHA: only test main branch on push.

Other branches will be tested when they have a pull request.
On #16 I saw 34 GitHub Actions tests,
half of them for the PR, and half for a push to the branch.
That is wasteful.

.github/workflows/tests.yml

@@ -4,6 +4,7 @@ name: tests
 
 on:
   push:
+    - main
   pull_request:
   schedule:
     - cron: '13 7 * * 0'  # run once a week on Sunday