ci: don't run CI twice for my own pull requests

`on: [push, pull_request]` has the effect that CI runs twice if I push
to a feature branch in my repo and then make a pull request – once for
the push to the feature branch, and once for the PR.

Instead, only run CI on pushes to the main branch (and on all PRs).

.github/workflows/test.yml

@@ -1,6 +1,12 @@
 name: Build and test
 
-on: [push, pull_request]
+on:
+  push:
+    branches:
+    - main
+   pull_request:
+    branches:
+    - main
 
 jobs:
   build: